I always saw Captain Toad as a likely candidate for Smash 4 DLC or, if it doesn't happen, as a newcomer for the next-gen Smash, because honestly after Lucina and Bowser Jr. I don't believe in series quotas anymore. If Sakurai wants to add a character, he will add it regardless of roster balance, simple as that. He won't say "Oh, I added a newcomer to X series over Y which deserves more characters, I must find someone from this Y series to add!" (The only possible exception is third-parties series never getting more than one character, but even then Sakurai will break it if he wants to)
Though I'd be okay about waiting for SSB5 to get playable Captain Toad in Smash. With two unique newcomers, one returning cut clone, stage over-representation (six on Wii U and five on 3DS! a total of seven new stages! also TWO Mario Circuits?!), and SEVEN new items, the Mario series is getting a bit too much love this time around I think. Captain Toad could be a cool character, but with how much Mario got in SSB4 I'd actually care about the series' "overrepresentation" even though it's a concept I usually don't take very seriously. Well, asides from KI:U Sakubias, of course.