Foreword: Time for the Raditz fight~ There are some changes but not too many as Raditz is still stronger than both Goku and Piccolo combined, just not as terribly. Let's start the show.
Piccolo stood behind Kame House to listen in on what Raditz was up to and it confirmed a few things he suspected and brought new facts into view. Goku being an alien was already something Piccolo had suspected but the fact that the tail he used to have could make him take a stronger form had been new. Goku having a son that also had this feature was also something new and a cursory sense of his energy showed that Goku's son was weaker than Bansuri, with the "highest powerlevel on the planet" remark being cemented by Piccolo's view of Goku's power.
'Seriously? Bansuri and I trained so much to beat you, thinking your strength had doubled since the tournament and it's barely grown at all?! If I knew you had slacked off on training this much I would have killed you ages ago, hell I could have gotten Bansuri to kill you and watched' Piccolo internaly admonished is rival, smirking when Raditz dropped Goku with a knee to the stomach before stealing his son and setting an ultimatum which Piccolo knew for a fact Goku wouldn't do and would no doubt try to fight him again.
"You'll never succeed, you're too weak" Piccolo dashed Goku's idea to fight Raditz as he made himself known in the air above them, having moved there to look down on Goku and his friends and freaking them out.
"Oh great, what else could go wrong today?" Krillin complained and sarcasticly asked.
"I suppose you could make me angry, but I wouldn't advise it" Piccolo answered, letting his power show and smirking at the fear simply relaxing caused.
"So, what do you want Piccolo?" Goku asked sternly, keeping a keen eye on his rival.
"I've been keeping an eye on your last visitor, I followed him here" Piccolo answered not liking what he was thinking about doing.
"Oh yeah? What do you know about him?" Goku rudely asked and staring defiantly.
"I know this much: the only chance you have of winning, is if I go with you" Piccolo stated, the sour expression on his face hinting his thoughts on the matter earning shocked looks from the revealed alien and humans.
Piccolo explained that Raditz was stronger than both of them alone and that the best chance they have to beat him is by working together. Piccolo also admitted that while he didn't care about Goku's son, he didn't want Raditz to take the world he wants to rule from him and the alliance is temperary, the next time see each other being as enemies.
"Alright, it's a deal and when the time comes I'll stop you again" Goku agreed to the team up, holding out a hand for Piccolo to shake.
"Heh, as if you could" Piccolo said with a smirk, shaking Goku's hand.
Goku got the Dragon Ball Radar from Bulma so he could find his son via the Dragon Ball in his hat and called Nimbus to fly quickly.
"Think you can keep up with Nimbus?" Goku asked teasingly
"Please, I'd be embarrassed if that toy could keep up with me" Piccolo answered with a surprising amount of humor before the two launched into the sky and flew in the direction of Raditz.
Raditz stood by his pod basking in the silence from putting the crying brat inside said pod. The brat certainly had a set of lungs on him, if Raditz could get the boy to stop screaming and actually train him, the brat could be useful..."If" being the main word in that idea. Raditz was about to sit down and get comfortable when his scouter alerted him to two powerlevels coming toward him at high speed. Raditz rolled his eyes and got back on to his feet in time to see the Namekian land in front of him with his brother jumping off a could to land after.
"Man Piccolo, you're fast" Goku stated in awe.
"Hmph, told you so" Piccolo said with a smirk before looking at Raditz with a serious look prompting Goku to do the same.
"Give me back my son and I'll let you go" Goku demanded, earning a laugh from the invader and a raised eyebrow from Piccolo.
"You'll let me go? Ha! didn't our last encounter give you a clue of how far above you I am?" Radiz mocked, a wicked smirk on his face before looking confused when his brother and the Namekian started removing clothes and their powerlevels shot up.
"Hmph, so you were hiding your true power Kakarot. 416 is better than the locals, but still the weakest here. Your Namekian friend has a powerlevel of 720 and I'm more than twice his strength" Raditz boasted his last sentance earning him shocked looks from Piccolo and Goku before they steadied themselves and shifted into battle stances.
There was a long moment of stillness when Raditz suddenly vanished from sight, Piccolo only barely avoiding the elbow to the back of his head with Goku being hit and sent flying, leaving Piccolo to face him alone for a moment with Raditz blocking and parrying strikes with ease before throwing out a powerful kick that Piccolo again only barely dodged before grabbing the leg and throwing him into Goku's kick to the gut which skipped Raditz across the ground only for him to vanish again.
Raditz reappeared behind Goku and grabbed the leg that had been used to kick him before throwing Goku into Piccolo, Piccolo leaping over Goku's flying body only to be punched in the face and launched back into Goku's prone body leaving them both in a crater.
"Owwww, that really hurt" Goku complained under Piccolo.
"Argh, and what's worse is that he's only toying with us" Piccolo voiced his own displeasure, getting off of Goku and standing up, not helping Goku do the same.
"Got any ideas?" Goku asked dusting himself off as he stood.
"I do, but you'll need to distract him for a minute and stop him from moving so it'll hit" Piccolo confirmed and explained, causing Goku to grimace before looking thoughtful.
"He's got a tail like I used to...Maybe if I grab it it'll stop him like it did to me?" Goku theorized, earning a nod from Piccolo.
"If it works, go for it" Piccolo said, getting a determined look and a nod from Goku before he leapt into the fray leaving Piccolo to put his index and middle finger to his head and start charging his attack.
"Ha! you think you can face me by yourself brother? The Namekian would have had better luck, but that's not saying much" Raditz gloated as he toyed with his brother, though Raditz was quietly impressed by how sturdy his brother was despite the fact he was fighting above his pay grade. Raditz was about to grab Goku's arm and make a 'why are you hitting yourself' joke when his scouter beeped, prompting Raditz to look and see the Namekian's powerlevel start to increase a worrying amount.
'800, 850, 900, 950 and it's still rising!? This could be bad!' Raditz thought and when he moved to throw and energy blast at the Namekian he felt a pressure on his tail and weakness throughout his body, forcing Raditz to his knees. Raditz looked back to see that his brother had grabbed his tail!
"Heh! Gotcha!" Goku said, keeping a good grip on the tail in his hands.
"Good work Goku now keep him still, It's nearly ready" Piccolo called out with a strained voice.
Raditz' scouter showed Piccolo's power continue to grow.
'1300, 1350, 1400 and still going up! That attack is going to kill me!' Raditz panicked and frantacly tried to get out of his brother's grasp.
"W-wait brother! Surely you wouldn't kill your own brother in cold blood, right? We''re family, we're part of the small group of Saiyans left! We could team up and beat the one who destroyed our race!" Raditz desperately tried to get his brother to his side, ot to at least let go of his tail.
Piccolo looked at the scene and smirked before remembering something Bansuri said:
'Goku is certainly a skilled and strong opponent, but it looks like he has a habit of dropping his guard after powerful attacks like the Kamehameha Wave or what he thinks is a knock out strike' Bansuri's voice ringing clearly in Piccolo's mind.
"Goku, don't you dare let that tail go! He's playing you to get free!" Piccolo shouted,hoping his rival had some sense.
"I promise to leave! I'll let the kid go and never come back! No harm no foul!" Raditz continued to beg, vividly aware that the attack being charged was stronger than him and still charging.
"...You won't hurt anyone else? And you promise to never return?" Goku questioned, making Raditz nod rapidly.
"That's right! I promise to leave peacefully and never return, just let me go! I don't want to die!" Raditz pleaded and to his delight he can feel the grip on his tail starting to loosen!
"GOKU DON'T!" Piccolo shouted, but was too late as he saw the tail slip out of Goku's hands then an instant later Goku's legs are swept from under him with a boot coming down on his chest with a crunch and a scream.
"To think you would be such a soft-hearted fool! My dear brother so weak he can't finish his enemy" Raditz gloated, twisting the boot on Goku's chest and drawing out more anguished yells.
"So, what are you going to do with that attack Namekian?" Raditz asked, continuing to hurt his brother.
Piccolo stood with his attack in hand, contemplating what to do. He could control the attack enough to make it curve around and hone in on his enemy, but they were fast enough to vanish from sight to cross the distance and strong enough to kill him. As Piccolo pondered, he picked up a power spiking in the alien pod, something the alien noticed too when the kidnapped boy burst out of the pod and destroying it.
Everyone paused and stared at the still crying child standing among them, in surprise at the power the kid showed. Gohan stared at Raditz in anger, still sniffling.
"L-LEAVE MY DADDY ALONE!" with a scream and another burst of power (that Raditz would read at a level of 1307), launching himself at Raditz and headbutting him in the chest so hard it cracks the armor and winds Raditz.
Goku, wanting to end this before his now unconscious son got hurt, forces himself to his feet and grabs Raditz in a full-nelson, locking him in place.
"Piccolo! do it now!" "What!?" Goku orders, much to the shock of the now caught alien.
"...I can work with that. Alright Goku, make your peace. Farewell! Special Beam Cannon!" with a final shout, Piccolo unleashes his attack (with the scouter reading 1872) right at his target, the beam easily piercing through both Raditz and Goku and sending them to the ground.
Piccolo smiles at the success of not only dealing with the invader, but also at finally killing Goku. Piccolo walks over to confirm the kills only to see them on their dying breaths.
"Y-you fool. You killed one of the strongest protectors of this world. Now it's doomed" Raditz rasped out, only to be shocked at the smirk Piccolo was giving him.
"Not really, his friends will gather the Dragon Balls and wish him back. He'll be back to annoying me in a few months at most." Piccolo smugly informed his fallen foe.
"W-wish? So you can have whatever you want?" Raditz probed
"Pretty much, Goku dying is an inconvenience at worst" Piccolo confirmed, getting a coughing chuckle
"Then my comrads will be here in about a year to make use of them" Raditz sputtered out.
"What!?" Piccolo looked shocked
"My scouter, it isn't just to read power. It's a communicator, and my fellow Saiyans heard everything that just happened. *cough cough* Your planet it doomed, my comrades are so strong *cough* I'm considered the weakling of the group, compared to those two" Raditz stated, shocking Piccolo to the core before coughing up more blood and dying with a sickening grin on his face.
"That isn't good" Piccolo muttered to himself and heard a vehicle fly overhead.
Piccolo watched the humans mourn the loss of Goku, the body disappear and heard Kami inform the humans that Goku is being taken to Otherworld to train with King Kai and prepare for the coming Saiyans and the time they're set to arrive. Throughout all that, Piccolo looked at Goku's child in contemplation.
'That kid has some serious power in him, maybe he could be of use against the coming Saiyans... No, while that power is impressive, the amount of time it would take to turn that brat into a fighter would be too long and I would be missing out on better training with Bansuri. I'll let someone else waste their time with the brat, I have better things to do.' Piccolo thought, deciding to keep an eye on the brat for the future, but not bothering to waste time with them for the moment.
Piccolo flew away from the humans and made his way toward Fortuneteller Baba's place to get Bansuri for more intense training. Piccolo figured the humans would find their own way to toughen up for the coming fight. He only hoped that the new trickes Bansuri learned from Baba would be of Some Help.