Y'know, I've been thinking about it.
If the Ice Climbers, Takamaru, ROB, G&W showed up, they'd likely have nothing to advertise for as well. It's not like Sakurai picks these characters to advertise things. It's merely a convenience for Nintendo if he has them ready by X date.
In addition, I've been thinking: Once again, it wouldn't have even made sense to reveal K. Rool yesterday, considering he himself would be anti-hype for TF (it's not like they could have said "Want to know more about K. Rool? Check out the website/go play Tropical Freeze!"
Therefore, it makes alot of sense for him to be revealed at E3 or something. You'll have a ton of gamers there, who know exactly who he is, and would be awestruck to see him. And besides that, all the hype for Tropical Freeze would've died down by then. (Hell, we might even get another DKC game announcement.) Perfect opportunity to drop the King bomb.
If we don't hear anything before release, he's also a good candidate for a secret character. Then again, so is Dixie, so if we don't get a new DKC character by then, expect the two of them to be fighting over it.