Take a look at how many new Fire Emblem characters were added to Smash 4 and Ultimate, and compare that to how many new Zelda and Kirby characters were added in those games. Just because a franchise has a new successful game or two doesn't mean it should definitely get a new character added, when the series is potentially already well represented.
I mean if we're talking about what
should happen, then it's hard to argue against Zelda getting more characters, and Kirby could use some more representation as well. In particular I think revamping the Zelda roster should be a priority next time. They're not the only series that could use more, though.
With AC in particular, the recent success is pretty overwhelming. Zelda's recent success is also pretty overwhelming. I expect new characters from both in some form, honestly.
On Fire Emblem, it has five completely unique fighters (not counting Roy here), which I think is reasonable; after that it depends on your stance on clones. Most likely, not every existing FE character will make it onto the next roster, though I don't expect a massive shakeup with FE outside of a few of its characters probably ending up low-priority and potentially getting a newcomer.
Regardless, nothing that's happened so far has completely blown up the concept of bigger series being more likely to get more reps, outside of very particular circumstances that throw things off.
Like for example, as was said above, AC and Splatoon have less right now because they joined later. Though honestly that just makes them pretty obvious candidates for roster expansion next time. Splatoon is of course a newer series than AC with only three mainline games, which may make the lack of multiple reps to this point a bit less egregious, but again both AC and Splatoon are due for something new in Smash 6.