I am wondering outside a bad timing case like Rex or a Gen 8 Pokemon what first party characters would get in? Basically would they be from franchises not featured in Smash or from ones that are in yet still with potential fighters to use, and in the case of both, what changed between now and 1-2 years ago?
I guess its a weird hypothetical for me that apparently such characters weren't a priority (or even mentioned in interviews as considerations) for the base game, but would be now. Don't get me wrong I'd love to have a Dixie Kong, Bandana Dee, etc, it just feels like a significant shift in some ways to suddenly include them in greater roles this much later.
I was wondering this myself
Because the Base Roster Newcomer Count was very low (on the Unique Side) It would be hard to get a Spot in the Roster. Dixie Probably wasn't in a Favorable Position (as a Non-Echo) since K.Rool was more of a Priority, and I don't think we would get 2 Unique DK reps in Base game
Though that is something weird. If they weren't Priority for Base Roster, why would they be for DLC, where you absolutely need to sell based on the Character alone and not with a Whole Package like the Base Roster Newcomers.
Though another Reason is Perhaps some of the Characters would be more Complicated to Make, whether it be in Moveset, or Needing a New Stage. If you Compare the Base Roster Newcomers to the DLC Ones, the DLC Ones have More Going on at Average in terms of Movesets. All of the DLC Newcomers also come with Stages, Which only 2 of the Unique 6 Newcomers did. Characters like Isaac and Rhythm Heaven probably fell on the wayside due to being new Franchises and Stuff, and perhaps more going on in Moveset (Especially Rhythm Heaven)
Looking at Suggestions for Base Roster like Isaac, Rhythm Heaven, Dixie, Elma, etc, I honestly Imagine these Characters having more Going on in Moveset, than the Base Roster Newcomers. Even Dixie if they want to Implement the Lost Tag concept from Brawl (The fact Dixie's Upgradeable Spirit is Dixie and Kiddy might add to this Idea). I Don't know about Bandanna Dee though, that might be a Case of Sakurai not being interested
As for characters not being mentioned for Consideration, I'm sure that not every character mentioned for Consideration for Playable is all that were Considered. Rhythm Heaven wasn't mentioned at all in regards to Smash 4 by Sakurai. We only know they were planned due to Datamine. I'm sure there were more than 4 Characters Considered for Ultimate that Ultimately Got Dropped