This was my thought exactly. Two ideas immediately came to mind. Either sackboy could do some sort of "special" where he kind of pulls the "costume" off of the character or when sackboy hits someone he absorbs their AP and it is somehow tainted or carries that character's signature. When this ap is absorbed, his appearance and abilities are altered. The latter would make it interesting because you wouldn't have as much control over who's powers you acquired.
How about once he takes their powers he uses their level 1, 2, and 3.. And he takes their powers by grapple hooking them and opening his poppet and customizing himself while the characters is tied down by his grapple hook
Honestly I could see two possible Kirby characters for this game. The first one would be, for me anyway, Musashi from Brave Fencer Musashi because he actually has the abillity to asimilate with his opponents abillities, although the does not change his costume. The other choice would be, of course, Sack Boy because of the costume thing. Out of the two I do believe that Sack Boy has a considerably higher chance of being a playable character.
"You know the carnival comes and goes. If you wait for a while, it'll always comes back to you..."