This one-shot was inspired by Be Not Afraid of Greatness by AshGopal
Here's the excerpt that inspired me:
"Crocus-san remained with Laboon and looked over everybody who passed through Reverse Mountain. He would determine who was worthy of his attention and advice and who was not.
Rayleigh-san and Shakky-san kept a close eye on Marineford, which was right next door to the Archipelago. They monitored the movements of the three Admirals.
Gaban-san was keeping watch over Raftel with a few others from the crew.
There was one of each of them stationed in the Blues to protect them and to make sure that the World Government didn't terrorize them too much.
Buggy was on East Blue. Already he had to overthrow a few islands where the Marines were tyrannizing the civilians. Granted, he may have done it with a too-heavy hand and ended up looking like the villain, but it didn't matter. He was a pirate, and this was Captain Roger's will. That was enough for him.
And then, Shanks was the Gatekeeper. He monitored the status of all the poneglyphs and made sure that no-one unworthy got a hold of all of them."
Disclaimer: I do not own One Piece and this is just a theory on what the Roger Pirates are doing and why
"Hey, Pirate King!" someone shouted up from the crowd. "Tell us where you hid all of your treasure? Is it in the Grand Line or someplace else? You found it didn't you…?
"You there, be quiet!" yelled the executioner, a fierce glare as he sweated even more under the noon sun.
"…the legendary treasure? The ONE PIECE!" continued the man.
Gol D. Roger laughed and said, "You want my treasure…"
The executioner interjected, "Just shut your mouth!" as the two hurriedly aimed their swords in hopes of shutting the notorious Pirate King up.
"…you can have it! I left everything I gathered in that place. Now you just have to find it!" stated Roger as he was stabbed and died with a smile on his face.
The crowd burst into cheers! The treasure was theirs for the taking. Anyone could set sail and find the treasure of the Pirate King. Which is bound to be wealth beyond anyone's dreams. This was the start of the Great Pirate Era.
Meanwhile, the man who had yelled those questions, put on a hat and made his way through the crowd, shimming this way and that to a nearby alley. He turned towards the platform and in a hushed whisper, he said, "Goodbye Captain, that's the first step down in the orders that you left for us. I'll see you later."
Then, Gaban took of his disguise, walked into the shadows of the alley, and snuck away to the harbor. Leaving to carry out his late captain's last orders, to watch over the One Piece and defend it if necessary.
Meanwhile, Rayleigh knew that he wouldn't be the one to help Roger challenge the world because how could he possibly stand there and watch him die. Best friends, captain and first mate, drinking buddies that have been together for so long and through so much.
And Roger knew this, so how could he do this. Curse that sickness, the only weakness of the man who invited/dragged him onto this whirlwind adventure. Curse him, Roger that fool who invited death like an old friend, who did what he wanted, when he wanted, where he wanted. Curse him and his last order. If he must live without Roger and his adventures, then he'd rather go back to living on a boat by himself, free and calming, but no.
He'll be on Sabaody, close enough to keep an eye out on Marineford without being in danger of getting caught. He'll track the marine admirals, watch their every move and pass along this information to the rest of the crew. He'll do whatever Roger says even if he's dead because he's the Captain and his best friend.
Meanwhile, Buggy stared at Rogue through one of his eyeballs. His devil fruit ability allows him to discretely check up on the Captain's woman. She's obviously doing fine, but she hasn't heard of Roger's execution yet, this island is somewhat isolated. With one last glance at Rogue's flat belly, which contains a child who will never meet the best father in the world, he leaves to go to East Blue where Garp's hometown is supposed to be.
If Garp does decide to help Captain's child, then he'd probably take them somewhere he can discreetly visit, so Buggy must go establish himself as one of the stronger pirates on East Blue to not attract suspicion from Garp. He can't be too overpowering, so just powerful enough to make due and patrol the waters that will shape the childhood of Captain's greatest treasure.
He may be the weakest of the crew, but that doesn't mean that he's weak. He'll play things cool, besides, it's not like he wanted to sail with that Red-Haired Bastard. Even he understood the feeling that Captain would be the only captain any of them would sail under. Placing his old hat aside, he carefully took up the hat he had designed in remnant of Captain's and placed it on top of his head. Time to gather the Buggy Pirates and set forth the plan that would span many years.
Meanwhile, Crocus watches the Den Den Mushi at the lighthouse. He tried his hardest to cure Roger's illness, but nothing worked. All he could do was prolong his life and what a life he lived. After coming back and informing Laboon of the fate of the Rumbar Pirates, he set about preparing for the vast amount of ships that would soon be heading his way.
This meant that he had to get the lighthouse up and running after his three-year absence. If only that blasted Laboon would stop ramming his head into the mountain, right where the stream came down. He was going to get seriously injured and even worse, he was going to stop most of the ships from crossing into the Grand Line. He'd better get a nice long vacation after this whole mess with the One Piece.
Meanwhile, Shanks watched as his fellow crew mate deliver the words that Captain had rehearsed before, but unlike all the previous rehearsals (because Captain wanted to get his last words just right), Captain was stabbed and killed. He was gone. All that was left was his treasure and their orders. He tilted the straw hat that Captain had given when he first joined and shed tears for the loss of the one man he saw as a father, a mentor, a friend and the only captain he would ever serve in his entire life.
Taking a moment to mourn for Capt- no for Roger, he then departed for his small dinghy to find a crew and fulfill his last promise. He had to get stronger, but he would have to do so alone. All alone with only his memories and the straw hat.
Beware marines! The Pirate King lives on in the ones left behind for they will find his heir, the future Pirate King because that is the will he left behind for the ones who didn't want to leave his side.