So… what series do you all think will only gain characters, and not lose any in a transition to a new game?
Well, it's safe to say that the most well-off series like Mario, Pokemon, Fire Emblem and so forth will struggle to hold onto every character in transition. Probably Zelda as well, which honestly might be a strong contender for this if Young Link didn't exist. But he does, so...
The safest series in transition - outside of the ones with literally only one or maybe two characters - are IMO the "middleweights". These would be relatively active or evergreen IP sitting at around three characters, namely Kirby, Xenoblade and Donkey Kong. Every character here feels uniquely important and popular, in my opinion there isn’t really any fluff to do away with.
But the other side of this question, gaining characters. I think there’s a chance Kirby and Xenoblade do, but I’m not fully convinced. Definitely some of the strongest competitors here but I'm slightly dubious of Bandana Dee or Noah / Mio's odds of making it in...
Oh right, Animal Crossing. Yeah, I think AC is actually the most likely to fulfill this with Tom Nook, since I’d be hard pressed to see a series this successful and active lose its only two characters so far. Which will eventually put AC in that midweight sweetspot of three essential, not-going-anywhere characters.