Honestly, my opinion of Captain Toad is this:
I'd far rather have him than both the Mario characters we have. I also think that he's quite likely for DLC. Even if you argue the notion "technically Captain Toad has only been around since Mario Galaxy," so has Rosalina. Of course, though, Captain Toad is THE main Toad from past games as well, just getting more prominence, so that makes him even more likely in my eyes.
...but with us already having both Rosalina and Bowser Jr., at this point, I don't WANT Captain Toad. At least not before we get King K. Rool, Dixie Kong, Wolf, Isaac, and maybe even another new franchise character.
Neither DK nor Wario are really Mario sub-franchises. Wario is a lot closer to Mario in general due to the fact that he's literally based on Mario, but other than that, Wario is a totally separate franchise. Same thing with DK, except even moreso. DK is not a sub-franchise at all; its a full franchise in its own right that has been very distinct from Mario, and DK characters rarely get added into Mario spinoffs gets except DK himself. Diddy's been added a bit more recently, but other characters are very rare to see, and even Diddy was now dropped from Mario Kart despite being in the past two console Mario Kart's.
DK is more of a "sister" franchise to Mario than a sub-franchise, with the DK franchise's lifetime sales being quite close to even Legend of Zelda, which is often touted as Nintendo's second most-important franchise (its not actually, Pokemon really is in that regard). Returns even sold more than both Zelda Wii games (not together, though), and DKC was the second best-selling SNES game.
Bayonetta arguably has a bigger shot at modern Smash than Krystal does.
If Nintendo buys the IP from SEGA, maybe. I definitely don't think Krystal has much of a chance, but I think Bayoneta is extremely unlikely as a third-party character.