In which case, almost everyone can be DLC because it's not hard to find a fitting stage and music for. The only thing you could even have trouble on is a batch of Spirits for series with an extensive Spirit list already, and even then it's not impossible.
I agree, but at the same time I think that some franchise will have a harder time make a decent fighter pack than other. Take Animal crossing for example, and let's say you want a Tom Nook challenger pack. Well you could add a New horizons map, but aesthetically, it would look very similar to Tortimer island or Town and city (of course you could make different layout and hazard, but it would have that same Animal crossing style). You could add New horizons music remixes, but most of New horizons ost sound very similar to the soundtrack of others Animal crossing, so there wouldn't be any super exciting music. For the spirits, you can add some of the new characters like Raymond and Judy, but they're not extremely popular to be honest from what I've seen. So in the end, sure you get new content, but nothing extremely exciting and nothing that really feel "new". And I think that's what the dlc are trying: each characters, stage, music and new spirits feel fresh.
Byleth did come from an already represented franchise, but Garreg mach was a new aesthetic that wasn't touched in Smash even with past Fire emblem stages. The music also sounds very different from what Fire emblem has done before. Sephiroth also represented a game that already have a stage, but they try something different by bringing the ending of the game (while most Smash stage bring the first area or the main area of the game represented). The stage feels very different to Midgar. And the music bring new remixes, which the base game lack in the FF7 universe. Those remixes convey a lot of emotion like Aerith theme or FF7 main theme that the 2 original songs couldn't.
So I think that instead of looking from new worlds, the question is "can the new character bring something new in this already represented franchise". I take the example of Tom Nook, because I think that New horizons isn't a ambiguous case: it probably wouldn't add a lot of new thing that Animal crossing didn't already do. But some franchise are in a more ambiguous situation. Take the example of Kirby. There'a already a lot of content, but also some more that is untouched. Would Nintendo think it's worth doing a Kirby challenger pack or they would think that Kirby is fine like this? I think this is more the question we need to ask ourselves instead of focusing on what's the definition of a new world.