Kirby's Pokémon Adventure would be an RPG / 3rd person adventure game for the Wii U and Nintendo 3DS. This is an idea I've been thinking about for a pretty long while, because I've always wanted to do a Kirby-related idea, as well as another crossover title (although I have done The Adventure of Mario and Link).
The concept for this game came to me as I was playing Super Smash Bros. Brawl one day and found myself comparing the basic overall mannerisms of Kirby and the different Pokémon fighters. As I did so, I noticed the incredible resemblance that the character of Kirby had to the Pokémon, and I wondered about what it would be like if Kirby ended up in the same universe and was mistaken for a Pokémon. This is the keystone element that brought the entire story together.
The game begins while Kirby is running away through Dream Land from a very large crowd of Waddle Doos (if you need to know what a Waddle Doo is, click here) and they chase him over the wacky terrain through most of the opening cutscene. Soon enough, Kirby spies a Warp Star in the distance that he can use to escape, but it is making a bit of a funny noise that's higher-pitched than usual.
As he is observing it suspiciously, King Dedede peeks out from behind a tree that Kirby passed by a few seconds earlier, and he chuckles mischievously. Kirby is suspicious about the star, but the Waddle Doos are approaching fast and Kirby has no choice but to grab it.
It ends up going haywire and swinging Kirby all over the place as King Dedede watches, laughing out loud at his prank. However, he is surprised when Kirby flies off into space on the star.
The scene cuts away to Squirtle, Pikachu, and Charmander, who are idling in a garden some distance away from the Battle Chateau in Kalos when they hear an explosion about 100 feet away. They run over to investigate and find a deep, smoking crater where Kirby landed.
Squirtle uses Water Gun and fills up the crater, causing Kirby to float to the top, with dust streaks on his face here and there. They lift him onto the ground, and then Squirtle and Charmander try poking him to wake him up. After a few seconds, Pikachu gets the idea to use a thunder shock to wake him up. This works perfectly, and gets the usual startled reaction from Kirby.
Kirby jumps away and they all face each other, after which they all start poking Kirby, trying to find out what he is. Kirby is bothered by this and to show it, he inhales Pikachu. Squirtle and Charmander are terrified, but then Kirby shoots out Pikachu and has the characteristic Pikachu hat on his head, which is emanating little sparks of electricity. Pikachu, meanwhile, temporarily has spirally eyes, but gets up and looks in astonishment at Kirby. The three Pokémon then lift up Kirby and sprint him over to the Chateau.
Calem, Serena, and Clemont (all from Pokémon X) are all there, and they are just as surprised by Kirby as the Pokémon were. Calem pulls out his Pokédex and points it at Kirby, which causes it to go all staticky for a few seconds, after which it flickers back up and gives this description:
Kirby, the bubblegum Pokémon. Kirby is able to puff himself up and float a limited distance, and can also inhale its opponents. Once it has inhaled someone, it can spit them back out or absorb some of their powers first.
Calem is incredibly excited at having discovered a new Pokémon, and he catches Kirby in a Pokéball.
From here, the story soon involves Team Rocket seeing Kirby in action and capturing him, which makes it necessary for Kirby to escape and return to the three trainers, with whom he has bonded slightly. King Dedede also gets worried about Kirby and travels to the world of Pokémon along with his minions, in order to try to find Kirby and bring him back home.
The gameplay for this game is inspired by titles such as Pokémon Rumble Blast and Kirby 64, particularly in the area of combat. However, its level layout would resemble the Pikmin series, despite its mechanics being different. The gameplay for the Pokémon characters would be just as you would expect, corresponding to the attributes of each different Pokémon. The three trainers would make use of their different Pokémon to pursue Team Rocket and try to find Kirby again. Kirby, on the other hand, would operate mainly by inhaling any opponents or Pokémon and adopting their powers, but he would have some resources of his own, like his Puff Jump.
To a great degree, this would be a game where you'd have to work your way through levels, impeded by some puzzles and obstacles that you'll need to solve through applications of special abilities of either Kirby or a Pokémon. It would be somewhat like a Zelda or even a LEGO game in this aspect, with the combat operating in a beat 'em up manner for the fights, except for most of the boss battles, which also implement elements of Zelda with the need to exploit the opponent's weakness.