Gonna be honest, I don't think we can even get a truly concrete idea of what newcomers are most likely to stay (besides the main stars or mascots of some series like Mario, Link, Pikachu, Kirby, etc) simply because it's too early.
We not only have no information on whether or not the next Smash game is even in development, but we're also coming off the cusp of the first and only game in the series to bring back literally every character that's been playable in a prior game in the series, in addition to adding a massive chunk of newcomers, many of whom are third-parties, over the course of the game's release and the DLC pack updates. We don't know how the next game is going to follow-up or change on that, or if it's going to be more than an enhanced port.
With that, and having limited information on Nintendo's next console, there's really no easy way to determine which characters are most likely to stay and which ones are most likely to be cut, with a few exceptions. We're in the period of speculation where more roster speculation is almost just pure guessing. Keep that in mind when considering both potentially returning characters and potentially cut ones.