Here's a theory for y'all that I've been thinking on:
Roster Diversity. I'm curious to see if this will ever be proven wrong.
Basically, ever since they started introducing newcomers one by one in Brawl, Nintendo has been very consistent on giving us characters that are different in terms of genre, appearance, ability, etc. when compared to the character that preceded them. Occasionally they will have characters from similar genres side by side, but, interestingly, it has never been more than two.
64 and Melee were obviously different in how they showed off their characters (they were just in commercials and the intros to both games if I remember correctly), but even then the one exception to this theory has been the Ice Climbers being confirmed after Peach and Bowser, but obviously they still differ visually. Maybe you could argue that having Shulk announced right after Robin and Lucina is a point against this theory, but I don't really see it that way due to Lucina being an echo and being announced at the same time as Robin.
If you don't believe me, you can just look for yourself at the roster:
View attachment 308240
The reason for this is probably obvious. Nintendo doesn't want you to get bored. They want to give us varied and interesting characters as much as they possibly can.
What can we do with this information? Well for starters, I think it's almost a guarantee that another JRPG character will not be next. It's perhaps even possible that we won't see a character like Dragonborn given he would be another sword user with magic (I'm not certain that Pythra actually uses magic, but it looks like magic anyway).
Not 100% certain on this of course. I just think it's worth keeping in mind.