For series such as Yoshi or F-Zero, it is entirely true there are not new viable characters. The inclusion of, say, Baby Mario or Kamek for the former, and Black Shadow, Samurai Goroh or Blood Falcon for the latter would be extremely forced; only for the sake of including another playable in neglected series. At what point do we stop? If so-and-so character were to get in for practically no reason, fans would jump on board and get the idea that it is now “fair” for another weak-arse to come up as playable status, and continue onwards from there. This is not the way to go.
Using a portion of Brawl as an example, look at three of the series that went from one representation in Melee to a greater number in its successor. Donkey Kong gained his pal Diddy Kong, Kirby was met by two friend/foe comrades Meta Knight and King Dedede, and Lucas joined Ness as Earthbound/Mother series major protagonists. Do you get the impression that any of these characters were thrown in for no reason other than a boost in numbers? Can this even be said for any character of the game?
It is easily accurate in saying to lean towards the lesser represented series when looking for newcomers, but it should not get to the point of getting forced, just because we “have” to add more. There’s a reason three of the four newcomers revealed so far have been from new franchises, the cast of returning series is getting strict for new arrivals. It ruins the cherishment of the roster to willy-nilly throw in a trend of a character from a single title of a big series.
It’s a manifest to the mind, and it’s easy to overcome reason when a person desires a single character so much simply for being them. The element of assist trophies is there for a reason. Space in the roster is limited; prioritise.