I think they absolutely have to give us some form of Expansion, whatever name they want to slap on that. I feel like calling it a Definitive Edition or something similar would be kinda poor form, just since the implies finality, and I'd rather see a game that can keep going for a few years... or just forever hehe.
I mean they just took the triple crown and all that. There are many people playing right now and having a good time with it. At this point, if peeps don't dig what's on offer at all, then of course they should move on. Like why waste the time on something that's not doing it for ya, but I suspect that lots of people will still kick around and float ideas for what they'd like to see. Or for general critique and criticism. Or I don't know, maybe these boards just end up like graffiti in the subway, but they could use it as a more of a resource if they wanted to.
I'd prefer just Baldur's Gate III: (with a subtitle) in the same way we had "Baldur's Gate: Tales of the Sword Coast" for BG1, or Throne of Bhaal after BG2, to wrap it up with a nice red ribbon like that, and give us a proper big ass interconnected dungeon crawler for the anticlimax. Then just a lot of support for modding to carry it along as far as it can go.
What made BG1/2 special to me was the customization. So the ability for the end user to import custom portraits, custom sounds and custom scripts. The base game was cool and all, but that was the stuff that gave it staying power.
Even after ToB, the sequel had a good 5 years running just off the novelty of Mods and playing the first BG1 game using the newer BG2 engine. If we ever get a BG4 from this same studio, then it would be particularly cool if they could do something similar here with the long view, since that's part of the charm. Import/Export Character, but once they call the curtain, then it's down to mods. That can either be an overwhelming all you can eat buffet, or slim pickings, depending on how hard it is to go under the hood, but that should get easier with time, whereas right now patches and hotfixes make exploring mods a bit rough. A year in the kitchen at Nexus has still been pretty impressive though. I've enjoyed the game at release much more than during the EA, but it's still behind what I had imagined in terms of party comp and char creation options, so more of that would top my list, but I'd really like to see more continuing adventures in this format with continuing adventures!
