That said K. Rool would obviously still be winning even if he hadn't.
But of the Nintendo characters Isaac would be in second, as he usually is.
This may just be my own biases speaking but
I do genuinely think that K. Rool, high in demand as he might be, isn't a character who would make it into the game. Let me explain:
Whereas Isaac would have a plethora of unique earth magic from plant life to stones to straight up energy attacks-- And then 3 other elements of Alchemy to draw from if they wanted to...
K. Rool just has things that other characters have already. He has cannonballs like Bowser Jr., He's big and fat like Bowser and D3, he has boxing gloves like Mac, Junior, or Killager, His tilts and normals would probably all be big meaty punches, again just like Bowser... Bombs like Link, Maybe a cutlass? Lots of characters have swords...
He's not really very unique. His tricks are all done already on other characters and better. In my opinion, K. Rool would be redundant. Just one more big Heavy slow character.
Inklings, although I don't think they'll make it into Smash 4, and may have to wait till next time for a spot, already have a myriad of other interesting tools and moves that would set them far apart from other Smash Brothers and Sisters.
My point, I guess, is that I think if it really came down to
A: A unique character beloved by his devoted fanbase
or
B: A character who's a mishmash of other mechanics we've all already seen
Sakurai would want to craft the more fun and interesting experience.
This is not to say that I know K. Rool would be that. Sakurai is a genius and has a real gift for making something we all thought would be stock and boring and turning into something interesting and unique. I just think that based on K. Rool's previous appearances in the DKC games, there's not much he's done that hasn't already been covered.