Kirby'd be able to fly via the warp star, of course. He'd have a default straight shot of just small stars, which would be a wider forward shot normally and a condensed shot while focused (holding a button to move more slowly and fire a marginally different shot-type), and also the Inhale on a different button that can be used on certain enemies. Inhaling enemies gives Kirby a Helper (it floats around you and attacks with you, a thing in a bunch of shmups I think, or at least it is in Touhou), which would be different depending on the enemy inhaled. Kirby's Helpers would each have unique skills, and here are some possible Helpers:
Fire: King Leo hovers in front of Kirby and shoots a flamethrower forward. Shoots a short-range wide shot while unfocused, shoots a mid-range slight spread while focused.
Ice: Chilly (whatever the snowman dude is called) orbits around Kirby and generates a circle of ice around himself, damaging nearby enemies. Floats near Kirby while focused, floats farther away from Kirby while unfocused.
Spark: The spark enemy that looks like a green Hershey Kiss floats in front of Kirby and generates electricity while the fire button is held, then shoots a lightning-blast forward once fire is released. Charging longer shoots a larger, stronger bullet. Same focused or unfocused.
Leaf: Leafan (the leaf enemy from Return to Dream Land, with green petals on its head) generates a swarm of leaves around itself while the fire button is held, then shoots the leaves in a spread-shot when fire is released. The spread-shot gets wider (with more leaves) with longer charge. Same focused or unfocused.
Fighter: Knuckle Joe fires rapid-punches to mid-range. Floats left and right while unfocused, nearly stays still while focused.
Paint: Adeleine (is my bias showing?) flings paint-blobs forward in wide spreads. Orbits around Kirby unfocused, holds still behind Kirby while focused.
Sword: Blade Knight floats up to the first enemy Kirby lines up vertically with and repeatedly swings its sword at them. Same focused or unfocused.
And for the shmup requirement smart-bomb, there are 3 options:
Mike: Has 3 stocks. Shoots a sound-wave in a large circle around Kirby, destroying bullets and damaging enemies. Weaker than Crash, but has more stocks.
Crash: Has 1 stock. Shoots a full-screen explosion. Stronger than Mike, but only has one stock.
None: Has 0 stocks. Lets you have an extra Helper (4).
You can have up to 3 Helpers at a time (4 if you have no smart-bomb). Swallowing an enemy to make a new Helper while you have the maximum number of Helpers replaces the oldest Helper with the newly-created one.
Bosses would be giant enemies like Whispy Woods (the first boss, as always), Kracko (this seems like an easy choice), Kabula/Kaboola (again, an easy choice), and then Meta Knight (who would be fairly small, like you, and give you a Sword-power before you start the fight), King Dedede (bringing the hammer down on that there Kirby), and Dark Matter (who would fight in Sword-guy form, then dark-blob form, then finally in giant-eye-of-bloody-death form). And for a true final boss, maybe Shadow Kirby (who would steal your Helpers before the fight (or just generate 4 pre-selected ones if you don't have enough) and split you into 4 Kirbys like in The Amazing Mirror, giving you 3 Kirbys as Helpers that just give you 4 times your standard shot for the duration of the sort-of-mirror-boss-fight) or Galacta Knight (because Galacta Knight), or maybe Susie (in her Robobot armor, summoning other machines).
For additional characters, you could have Meta Knight (who would move faster than Kirby, would have an unfocused shot of shooting swords straight forward, would have a focused shot of short-range sword-slashes, and would have a Blade Knight Helper) and Ribbon (whose shot is shooting a spread shot of crystals, and she'd have a pair of crystals that shoot other crystals straight forward, playing more like a standard shoot-em-up character).
This would never, ever happen. But I just had to get it out there.
See? Kirby can be fit into ANY IDEA