Maybe so. But I just don't think it's likely. Even Poochy seems more plausible to me. Besides that, we already have 2 Mario universe villains. Bowser Jr. could very well stand in for Baby Bowser in concept. I also really doubt Yoshi's Island will get a second character ever, much like F-Zero.
I know you don't think he's likely, but the criticisms you're making aren't answering my question of why.
Poochy, while well enough known, frankly isn't recognized as much as Kamek is. Further, he doesn't seem to do much that could be naturally used to make a full moveset that still feels interesting.
Further, while it's true that we don't
need another villain from the Mario universe, that is a lack of a positive trait rather than an inherently negative one. We also didn't need six Fire Emblem protagonists or six third party characters, but we got them.
In fact, the same argument could be used for the DK series against K. Rool. After all, the Donkey Kong franchise is part of the Mario universe, and we don't need another villain from that world, right?
The Yoshi franchise doesn't need another character, sure, but that's not enough to discount the character. You especially need to consider that the Mario universe is Nintendo's flagship. Adding more characters from that world makes perfect sense given how far-reaching and beloved the franchises under its umbrella are.
We could get Toad, Captain Toad, Paper Mario, Waluigi, Daisy, Kamek, Ashley, Dixie Kong, Cranky Kong, Funky Kong, and King K. Rool and all of those additions would be deserving.
Each one is well known and loved, and I guarantee more people would recognize them than would recognize Ness or Captain Falcon or Shulk or Pit or Olimar. The character has credentials all his own, and being another villain from the Mario universe doesn't hurt him in the slightest.
You say the the Yoshi series is about as likely to get a newcomer as the F-Zero series is, but you've yet to give a clear reason why.
The Yoshi series has 13 games to its name and has likely sold multiple times more than the Mother or Xenoblade or Kid Icarus franchises. (Sales data is not all released, but we at least know that Yoshi's Island sold 4 million copies.) It has had games releasing consistently since the SNES and it's even due to get another game soon. In the time period Sakurai was likely looking at for roster selection, the Yoshi franchise had
three releases. And that's not even counting Kamek's appearances in the Mario franchise.
So, allow me to reiterate my question. Aside from the
assumptions that only popular characters get in and that Mario sub-series are doomed not to get more characters, why is Kamek, a recurring and recognized villain (I'm confident enough to say more of the casual modern audience knows him than K. Rool) from a popular and growing staple franchise of Nintendo so unlikely?
I want you to give me something that's more concrete. Popularity alone isn't the decisive factor in what characters get chosen, and there's no more reason to assume that the Yoshi series can't get a newcomer than there is to assume such for the Donley Kong series.