I'm currently not convinced big titles would risk trans women companions, though. We're more likely to get trans men because they're not as "scary". The TERFS hate them less and so do people who would feel... violently angry encountering a "trap".
No, I can't see a game like BG3 doing a trans character full romance of any gender for a while yet, so I don't think that's a slippery slope that anyone needs to be concerned about.
I guess there might be a semi-equivalent to a playersexual approach where a companion could be either cis or trans in the same way as a playersexual character might be gay or straight. Though playersexual characters can be bi or pan, they are not necessarily so just because they'll fancy PCs of different genders in different playthroughs. I think of different playthroughs as different possible worlds or alternative universes, and on this proposal the romantic interest would not be "really" trans or indeed "really" cis, but could be either depending on the world the player created through their choices. But I'm not able to judge whether that would be an appropriate representation of a trans character in an romantic context, as you say that's something that trans people themselves would need to comment on.
And it would no doubt also put up the backs of other players who don't like to think of even alternative universes in which their romantic interest is trans. I think there's zero chance of BG3 going this route, and however much I'm in favour of trans representation in the game I'm not sure I would even advocate for it. The (real) world doesn't feel ready and it's probably more productive in the long run to take those baby steps you mention and, as you say, get the concepts into people's minds before doing anything that would be seen as so central to a game.
Possibly one way that RPGs could start to introduce romances with trans or other TQIA+ characters, though I don't for a moment think BG3 will do even this, is by having non-party NPCs that our characters could meet in the course of their adventure, chat with and possibly chat up and have at least some romantic dialogue with (or no strings attached sex if that's what all parties want!), without this being one of the "main" romantic arcs of the game. I do hope that BG3 will take this approach to add some additional romance options with, e.g. dwarf, halfling and gnome characters or just some different choices than the companions we adventure with if none of those seem right for the character we're RP-ing. They don't need to have a huge amount of content or any big romance scene animation, as imagination can go a long way. But as I've mentioned already in this thread, having at least some in game recognition to hang your head canon on can go a long way!
Nonbinary people do not appear to have been included in this game.
I'm not sure it's enough to constitute non-binary people being in the game, but you can select a gender identity for your main character of "Non-binary/other". It makes no significant difference to the game other than on the few occasions anyone refers to you by your pronouns they use "they/them". Well, there are currently some places where an NPC will still use a gendered term such as "lass" to refer to the character, but I assume these will be removed and are only there because Larian's search for "she/her/he/him" didn't throw up these cases on their first pass at adding an additional gender option!
I'm not sure whether this would be the sort of thing you'd consider a last minute fix that you'd rather do without. It has so little content that it feels as though players could project almost any (though not every) enbie identity onto it. As a cis person, even though it's not much, it's enough to get me thinking about enbie character concepts that probably wouldn't have occurred to me had that option not been there, and I do have an idea for a gender fluid warlock I'm excited to play. But you're of course right that I'm not the most important audience here.
