Now, this is operating under the assumption that these are realistic decisions and not my own pipe dreams. Course, I am HELLA BIASED so deal with it.
-3 Nintendo newcomers,
Isaac (Golden Sun), BWD (Kirby), Rex & Pyra (Xenoblade Chronicles)
Isaac and BWD have been long, LONG requested with many fan requests and has remained a consistent want for a dozen years now while Rex and Pyra have been recent mega-hits and a representative of the continuation of a large-scale franchise which Nintendo could not possibly overlook. From a popularity and advertising standpoint, Rex and Pyra are a no-brainer.
-3 3rd party,
Jack Frost (Atlus), Ryu Hayabusa (Koei Tecmo), Doom Slayer (DOOM)
All these 3rd parties have had a large amount of historical significance to Nintendo and in gaming as a whole. At the same time, they're the faces of their own respective genres and would be recognizable to most people who are even somewhat aware of videogames. As an aside, all of them have recent games headed over to Switch, also making it a prime advertisement opportunity. As much as I'd prefer an actual protagonist over Jack Frost, there's no denying he's far more recognizable, which is also the only reason I think Nintendo would pick him over Erdrick from Dragon Quest while Slime has a bit less to work with in terms of moveset potential due to needing to draw from variants to be unique.
-3 upgraded assists
Lyn (Fire Emblem), Bomberman (Bomberman), Waluigi (Mario)
All three have had an extremely large amount of fan-requests over the years, on top of being parts of still relevant franchises, with one in particular causing a large influx of jokes with his deconfirmation. Considering none of them are lacking for moveset potential, it wouldn't be so difficult to create a unique moveset as well. if any assist trophies in particular were getting cut and re-added as fighters, I'd find it hard to see any other assist trophies being as most likely for a DLC upgrade.
- 3 echo fighters
Octoling (Splatoon), Shadow (Sonic), Medusa (Kid Icarus).
Shadow and Octoling serve as both rival characters and as villains, mimicking the player character's abilities, albeit with a different visual flair, a relatively easy way to make them lazy echoes. Shadow would have to give up his chaos abilities and guns, but echoes don't exactly adhere to lore to begin with. Medusa I see happening because Sakurai Bias but also because she's played a major role both in the retro game and in the reboot while having similar proportions and weaponry as Palutena except with a unique appearance and visual flair. (Except for the shield, but they could easily just make it a dark shield or something)