The Switch was released in March 2017, and after the write-off that was the Wii U, they probably intend to go headstrong sometime into the early 2020's, likely between '22 and '24 (though it could be later if the Switch gets a pro version). I think the Switch was planned to be the second coming of the Wii, so only releasing content for undoubtedly their standout title of 2018 and one of the defining games of the Switch up until early 2020 seems... unlikely to say the least. If they do happen to ignore the game thereafter, it'd probably be the worst route Nintendo could take. I firmly believe a more Splatoon 2-like approach will be taken with Ultimate, in the sense that the game will receive more and more content with time. Practically all of the mini games were completely disregarded this time around, and to avoid having single player become stale, I think the return of certain minigames as either free or relatively inexpensive is iminent.
2018 was an intermediary of sorts where the big titles were released the year prior, and a BUNCH of big titles are planned for this one. What's more, the two "saving graces" for this year, Star Allies and Tennis Aces, ended up tasting a bit bland for most (in fairness, Star Allies was a late bloomer with the content packs, but it's still so easy you could breeze through it in at most 1 or 2 hours more), with non-Nintendo releases like Octopath and Dead Cells reeling in all the profit. Smash was released in the latter of 2018 and may very well just be considered a 2019 release from hereon in with how late it arrived. It exceeded 5 million sales globally in the first week alone, and with the holidays so close, I'd assume it's a safe bet that sales reached at least 7.5 million by now. Odyssey sold 2.2 million in three days for comparison, so Ultimate is definitely a frontrunner in sales. With its release date, content, and popularity in mind, how truly feasible would it be to only release 5 DLC characters? On top of that, how would it make sense to release them in 2020 if the Switch is probably going to outlast the game by a mile? The way I see it, there's more in it for Nintendo if they add more content than the Fighter's Pass, and I have a feeling the Big N knows that, too.
What I'm trying to say is that in no reality will Nintendo purely release the Fighter's Pass. A massively successful title on a console that makes it exponentially more accessible is probably not too content with a mere 5 characters. Even if it's just one or two more characters, it's just an undeniable profit, though I'd have to imagine that a standalone character would have to have more reason for inclusion than those in the Fighter's Pass. Someone who people have asked for since Brawl, and to this day maintains a large enough following despite having their last major appearance in a game nearly 25 years ago. Hmm... I wonder who it could be...