The majority of these DLC characters have felt like a promotion in some way, shape, or form to me. Whether they're an ad for the Fighter's Pass itself (Joker, Terry, Banjo) or for games on the Switch (Byleth, Hero, Min Min). Some characters are chosen because their games, or the characters themselves, are popular which means more people are going to be more inclined to buy the pass - and others are chosen because they want to promote other games to sell us. They wanna draw new people in, and get the people already drawn in to spend more money on other things.
I honestly don't think there's much more too it than that. Maybe at one point Smash was a way to celebrate gaming, (maybe the base roster, or Spirits and the like, of a Smash title is better suited to that these days) but lets face it; not with Nintendo at the helm of (DLC) character choices. They're just not going to pick some rando unless they have a big fan base from which Nintendo can extract cashmons.
The only outlier of the six pass characters so far, in my opinion, is kiiiind of Terry. But that's mostly just because I don't remember him being requested, and I'm not familiar with his series enough to claim anything one way or another regarding his inclusion. My guess is there's a lot of KoF fans and Nintendo wanted to spread another tentacle sucker in that direction. Everyone else - even someone like Joker who was a complete surprise - makes total sense to me. He wasn't a Smash fan requested character, but Persona is a big name and has been having a good show of it lately, I'm sure Nintendo just wanted to draw in that crowd of rabid Persona fans.
I guess the moral of my rant here is I don't subscribe to the notion of "Smash is a celebration of gaming!" or "Publishers/Developers all need a rep!" or whatever. Nintendo don't give a hoot, a holler, nor a poop about any of that. They just want our money, and if our money is tied to popular or historic characters then great, they can send their suckers to feed at their leisure. If not, oh well, they've got bigger fish to fry.
If it were still Sakurai deciding DLC characters, I wouldn't feel this way. But I think the days of Smash being the child of Sakurai alone are over.
All that said, it bodes well for Isaac! If he's getting a new game, and he already has a big fan base, then he kills two birds with one stone! Should be a lock!
On another note, I know I'm biased, but Fire Emblem having the characters it has isn't that big of a deal. I wish their movesets were more unique, but otherwise it's not the end of the bloody world. If Final Fantasy was a Nintendo only thing we'd be seeing a bunch of those characters instead. Not only that but if Golden Sun was the go-to JRPG of Nintendo we'd be seeing Isaac, and Felix, and Matthew, and everyone would be ******** about "those orange/brown haired sword fighters!" It's just one of those franchises that has a new cast every game or so, and Ultimate happened to have everyone. They would have dropped half of them otherwise. New game means another opportunity, because Nintendo wants to sell games and DLC for that sweet sweeeet greengreen! They're doing the exact same thing with Pokemon. I feel like it just helps solidify my point.
On an unrelated note: I'm replaying Golden Sun. It still holds up! I just got into Xian. I plan to 100% it.