Prince Fluff visits Planet Popstar!
The idea behind this is either the sock that Prince Fluff has starts acting weird or Kirby sends a SOS note through the sock, so Fluff goes in to investigate. This would lead to him helping and/or saving Kirby.
He would have some getting use to the whole 'not being yarn' thing and have to find a new way to fight and navigate this world.
Since Fluff would be good at manipulating yarn, he thought it would be a perfect weapon.
He sports a spool of pinkish-red yarn (which could be other colors) on his back with a seemingly endless supply of yarn. He uses two metal knitting needles as his main source of attacking.
I picture his playstyle would be like a mix of Kirby's spear and spider copy abilities. Where the needles would be the spears, but dual-wielded so a faster attack combo and the yarn working like the web. It could also sport some similarities to Kirby's ice ability like hitting a boss enough times could "freeze" them in place for a second, 'cause they're wrapped in yarn. Or wrapping up a small enemy in yarn, similar to Kirby's Epic Yarn, and chuck them.
I also don't think Fluff can hover like Kirby does, so he would have to throw one of the needles into a wall and climb up the yarn line. I think he would have a pretty high jump though to compensate for the lack of floating.
One more thing: I don't think he would have to white background on him, Normal Kirby is just drawn like in the game and I thought it would look nice in this drawing.
Man, I love Kirby's Epic Yarn. And Prince Fluff! He's a cool guy that deserves to be in another Kirby game.