The comments from Larian after doing BG3 lead me to expect they want to work with IP they control, not that they'll go do another company's property.
Yeah, Swen was pretty clear that neither of the two games they're currently developing is based on an IP owned by anyone else.
So that rules out an actual Fallout, Ultima, or the like. I think it's superficially unfortunate because I think Larian being more restricted setting-wise would probably do them a lot of good, but I hope to be proven wrong because they have much, much stronger writers now, none of whom were on board in when DOS2 was developed.
All that we do know is neither game is DOS3, and at least one of the two games is a CRPG (specifically a CRPG, not any other kind of RPG), and the scope on both is smaller than BG3 but that's like, still potentially fairly huge.
It'd have to have Bethesda give up the reins, which does seem exceedingly unlikely.
Bethesda could licence it to them like they did with Obsidian and Fallout: New Vegas, but I think it's extremely unlikely for three reasons. Firstly, Bethesda/Zenimax management are incompetent and short-sighted (c.f. the last 14 years), and would probably just never even consider this off their own bat, as much as Todd Howard might occasionally maunder on about how good New Vegas was (not his tune at the time). Secondly, I don't think Microsoft, who could make them, are currently on-the-ball enough to make it happen, and whilst they're getting less cautious about letting their IPs appear on non-MS platforms, they're not quite there yet. They also made a frankly insulting and desultory offer to put BG3 on Gamepass, valuing it at a paltry $5m (which is like, much less than 1% of what its revenue has been, probably less than 0.5% at this point), which I doubt impressed Swen much and surely didn't show much business savvy on MS' part. Thirdly, for the reasons Swen stated, I don't think they're keen to try and use someone else's IP.
I do think Larian would do very well with a post-apocalyptic setting, because it would fit well with their love of grimdark and jokes and outre characters, but I suspect it'd be their own one.
Which for that to happen, Larian would have to both (a) buy the rights to the Ultima series as a whole from Electronic Arts, and (b) either get permission for or buy the rights to the certain elements of Ultima that Richard Garriott still owns.
I think this is one of the more plausible future scenarios for
a future Larian game, but given Swen's intense love of Ultima (one I share), I don't think he'd be able to, or even want to, contain himself if he'd managed to actually buy the Ultima licence. Also AFAIK EA have never sold a "vaulted" licence, no matter how worthless it was to them, because they're horrible twats. They don't even licence them out, generally (I can't think of an exception in the last decade). I do think double-digit millions might make them change their mind, because Ultima is a dead brand, but really, if Swen had managed this, we'd know about it. He's not a "games must be secret until 6 months from release!" kind of guy. Likely the only reason we don't know about the current pair of games is because they're in flux and may be cancelled (Larian have cancelled quite a few games in the first third of development).
Anything else they do will almost certainly be less successful, but that doesn’t mean the decision to move on was a mistake. Their lack of enthusiasm for staying in the D&D milieu, as well as the near impossibility of catching lightning in a bottle twice, means it makes sense to move on.
To be real, I don't think they caught lightning in a bottle here. I think pretty much everything about BG3's success, once Larian stopped trying to force BG3 to be DOS3 (which they most assuredly were initially doing, in the beta), and was basically pretty predictable.
They made a massive, high-quality CRPG, when gamers hadn't seen an AAA CRPG for what, 13 years. They had tactical combat system which was pretty engaging, and turn-based party combat has been well-established as fairly popular for a long time (though it is still less popular than Mass Effect/Skyrim-style, interestingly, but that's a separate discussion). They created an extremely engaging cast of highly dateable characters and put in significant dating sim elements. They put in a ton of effort and revisions and polish and polish and polish and polish and revisions and polish to the dialogue, to the point where they were doing it post-release, something even Bioware have never done AFAIK. They'd also laid the groundwork for that by realizing they didn't have good writers, despite the fact that they had beloved employees doing that, and instead of firing them or something, hired above and around them, and made sure they acquired good writers. This is something most game companies are just unwilling to do. They're too precious.
Now, it making $1bn+ (maybe a lot + now) was unexpected to a lot of people, but you had to be a little shortsighted to not expect it to make hundreds of millions, to not sell 10m+ copies.
I think by following the same formula, Larian could pretty much definitely have made another very big hit - maybe even bigger than BG3.
However, I don't think they enjoyed working on someone else's IP - even though I think it was doing them good to be restricted in that way - and I think that plus six years of working on a single game meant it was a very valid decision to not keep going that way. Especially as WotC would likely have been greedier this time and probably demanded more control.