Honestly for Additional DLC I feel like they already blew most of their big new franchises out with the original Fighters Pass so I wouldn't be surprised if the Additional DLC was like Base game which had a good blend of new and old 1st and 3rd party franchises. I could see us getting someone like Dixie as an Echo or semi-echo which would represent an old already represented franchise while we get someone like Crash Bandicoot and he gets a Stage.
Kind of like Smash 4 DLC where some had a new stage and some didn't and some were from already represented franchises while some weren't. The "New Worlds" comment was a mistranslation after all
so as I said I could very well see a blend of new and old franchises but not only just new 3rd party franchises but new 1st party franchises as well.
If Assists and Spirits can be upgraded which I think the latter is almost certainly on the table I could see us getting some new 1st party franchises: Rhythm Heaven, Golden Sun (Hopefully), Advance Wars, A new retro rep (Takamaru maybe/Lip/Mach Rider), Etc. Bring it on Additional DLC cause this ride isn't stopping any time soon!