Being relegated to boss status is one of the worst things that could happen to K. Rool. Not only would it be incredibly awkward to see him several times Bowser's size, but it's just not a fun role for a character to have. If you really like K. Rool as a character, it just wouldn't be much fun seeing him repeat the same telegraphed maneuvers over and over again, for all eternity. Smash bosses, especially stage bosses, like @
Zynux
said, have highly limited AI. They come off more as mindless animatronics than the characters they're supposed to be. K. Rool's original SNES boss fights were much like this, so I can imagine a boss role being appreciated as a nice throwback for about a week before everyone starts wishing he were playable instead.
I'd rather see K. Rool appear as an Assist Trophy. At least then, he'd have a little more character and would seem to be more involved in the actual fighting. If, by some chance, he ended up being a boss but had a lot of unique animations and personality quirks, I'd probably prefer him in that role (since he was
created to be a boss, after all). But if he simply ended up like the Yellow Devil or Brawl's Ridley, I'd rather he be spared of that fate.
To weigh in again on the stage debate, I'd much prefer Gangplank Galleon to the Flying Krock. Sure, we already have (and have had) a pirate ship stage with the Paper Mario stage and Pirate Ship, respectively, but we also have (and have had) a flying fortress of villainy stage with Halberd. My hope is that Gangplank Galleon shows up on the Wii U version, so that its aesthetic doesn't have to conflict with that of the Paper Mario stage on 3DS.
Gangplank should pull elements from both DKC1 and DKC2, for the record. Have it be a stage with phases, like Castle Siege. The first phase takes place on the ship's masts, with rope ladders and crow's nests for platforms, like in the opening levels of DKC2. Then the players fall down onto the deck, which would resemble K. Rool's arena from the end of DKC1, complete with cannonball rain as a hazard. Finally, the floor of the deck caves in when a canonball falls down and breaks through it, causing the fight to transition into the confines of the ship, which has wooden crates serving as platforms and rising, swimmable water (such as in Lockjaw's Locker).