I don't think it's very likely for any modded NPCs to join the vanilla game - remember CA initially did every part of this game on his own, over years. Just randomly adding someone else's entire NPC, however well-made, seems pretty unlikely, especially since in order for the NPC to make sense in the mod, everything about her (custom locations, events, dialogue, etc) is based on her living in SVE with references to SVE content.
I think that's a good thing, though! Consider that it's pretty easy to mod Stardew Valley and play with all the custom NPCs you could ever desire, so when we get game updates, it's not just recycled content from mods you've already played, but things the dev thinks would be fun and new on their own, unhindered by what people have already done in mods, and able to make changes to the game wherever necessary to support it.
Meanwhile, mod authors get to just tinker around and make what they want, when they want, without handing over content to an official team on a slower update cycle. You get the best of both worlds :)