Based off a suggestion by SarcasticInkling.
"So," Pikachu said in between licks of the ketchup he'd pilfered at breakfast. "you got into Harry's dream once, right?"
"Yeah." Ash nodded, glad that he was the only one at the table who could understand pokemon. He hadn't told anyone other than his trusted partner about the incident, and didn't plan on doing. If he wasn't careful with his words, he could end up accidentally admitting it to everyone at the dinner table.
"Do you think you could do it again?" the mouse asked. Ash shrugged, thought for a bit, then nodded. He looked deliberately over to his friends, who hadn't yet caught onto the conversation - Ron had scarfed his food just a little too quickly, and everyone was trying to stop him from choking - in lieu of responding verbally. Pikachu nodded knowingly. "So, could we try it tonight?"
Ash nodded again, smiling at the thought. 'We'll have to do it after everyone else goes to sleep, though...' he decided.
It wasn't long before the boy's dormitory was filled with sleeping students. Ash, who was far too excited to go to sleep anyway, turned to Pikachu, after testing whether the other boys really were asleep or not.
"Okay," he whispered. "I think you have to go to sleep first."
Pikachu nodded sleepily. "Can do!" he hissed back, curling up on Ash's pillow. After a moment, his breathing deepened, and the mouse started letting out small snores.
Pokemon asleep, Ash reached out to him with his aura. Glancing at the surface was easy, and he put a quick extra burst into it to push him into Pikachu'smind.
"So..." Ash said, opening his eyes to a field like the ones surrounding Pallet Town. "What now?"
Pikachu, who had previously been lying down in the grassy expanse, looked up at his trainer and beamed. "Hi Ash!" he greeted. Suddenly, the pokemon was sat on his shoulder. "So, it worked?"
Ash nodded. "We're officially in a lucid dream. Cool, right?"
"Yeah!" Pikachu agreed readily. "Wow, we can do whatever we want now..." the mouse frowned, "that's way too much choice!"
"Well, I think you're supposed to do stuff like flying," Ash informed. As he spoke, the grass below them disappeared, replaced with clouds, and an otherwise clear sky spanning as far as he could see. Ash grabbed Pikachu's hand and shot off, arms spread eagle as they usually were when caught in a legendary pokemon's psychic.
Pikachu giggled at Ash's flying. "Hey, look, I'm Super-chu!" he grinned, pushing one arm out as far as he could stretch it and bringing the other close to his shoulder. A moment of concentration formed a cape around said shoulders, and a mask - red to match the pokeball pattern on the otherwise white cape - around his eyes.
"Coo!" Ash enthused, stopping his flight to admire Pikachu's new getup. "So you can make anything you want, huh? Well, I want... the biggest burger ever!"
A burger promptly appeared in Ash's hands, and the trainer took a massive bite out of it. He licked his lips appreciatively, mumbling "Tastes good!" around his second bite. Taking Ash's lead, Pikachu appeared a bottle of ketchup.
"So," the electric type said once Ash had finished his dream meal, "What do you normally dream of?"
Ash brought a hand to his chin contemplatively. "Probably... pokemon battles!" he announced. A battle court took over the sky, with a selection of pokemon on the other side. "Against legendaries!"
Now, the other side of the field only had one pokemon: Lugia. It moved into the lines, levitating on a lower tilt in preparation for battle. With an enthusiastic cry of his name, Pikachu joined the legendary within the lines.
"Okay, Pikachu: use thunder!" Ash commanded, pointing out to Lugia. Piikachu complied, taking a second to build up charge before releasing it all in a massive yellow bolt of electricity which engulfed the beast of the sea. Without even having a chance to respond to the attack, Lugia succumbed to Pikachu's power.
"Yes!" the pokemon shouted, pumping a fist into the air. "I love this dream! Way more exciting than the usual ones..."
"Oh? What do you normally dream of?"
Pikachu rubbed the back of his head sheepishly. The grassy field returned, and Ash found himself sat cross-legged with Pikachu in his lap. He scratched the mouse's head absently.
"So?" Ash prompted.
"This is what I usually dream of!" Pikachu revealed. "And sometimes..." A mecha hand moved out of nowhere, knocking Ash out of the way and grabbing Pikachu in one swift motion. 'Team Rocket's in our dream too?' Ash thought, startled.
"Sometimes we get to beat up Team Rocket together!"
"Prepare for trouble!"
Ash blinked. The motto's words were familiar, but that wasn't Jessie's voice...
"And make it double!"
That wasn't James, either! The motto - Sinnoh's, if he recalled correctly, continued as Ash gave chase to the retreating machine. Finally getting close enough to see inside the glass bubble on top of the robot, Ash looked up to see who was controlling the mecha, seeing...
"Snape!" the potions master, dressed in Jessie's usual uniform, complete with purple wig, shouted.
"Crabbe!" One of Draco's followers had taken James's place, with a short lilac wig and rocket uniform.
"Malfoy, now that's a name!" The Slytherin boy was wearing a pair of cat ears and whiskers, but otherwise had his typical uniform.
"Putting the do-gooders in their place,"
"We're Team Slytherin -"
"In your face!" Draco finished, giving a V for victory pose, complete with wink.
"...What." Ash deadpanned. He managed to keep a flat expression for all of five seconds, before bursting out laughing. "Pikachu, use thunderbolt!" he managed to shout out between laughs. As the pokemon fired its attack, 'Team Sytherin' changed expressions from affronted to terrified just in time for their usual blast off.
Pikachu was sent barrelling into Ash by the explosion. He landed in his trainer's arms, also laughing hysterically. "That was great," he said, wiping a tear from his eyes, "the best dream ever!"
Ash, for his own part, was too busy laughing to respond. Just the mental image of the Team Rocket impressionists was enough to keep him laughing. He flopped back, barely able to breathe he was laughing so hard -
- and landed back on a mattress. Ash took a deep, steadying breath - who knew you could wake yourself up from laughing too hard? - and attempted to relax.
A groggy, frowning Ron shuffled over to the trainer's bed. "What're you laughing about?" he asked, almost accusingly, squinting at Ash and Pikachu.
The two looked at each other, and burst into fits of giggles all over again.
"Mr Ketchum."
"I will have to remove you from the classroom if you cannot control yourself."
"Mr Ketchum!"
Ash looked over in the direction of the Potions teacher - just his luck to have Potions today, of all days - unable to quite manage to look him in the eyes. "Sorry... sir..." he choked out, breathless from laughter. "I'll just..." he took a deep breath, face straightening.
"Team Slytherin," Pikachu whispered into his ear, and Ash's neutral expression twitched into a grin. Before he could go into full-out laughter again, he dove out of his seat and out of the door. As it slammed behind him, the rest of the class heard his laughter, muffled by distance and the stone walls of the castle.
Iris looked over at the source of the noise, frowning in confusion. "What's up with those two?" She glanced over at Harry and Ron hopefully, but the two boys simply shrugged.'