Bioware is certainly AAA mainstream. Like all other mainstream developers, it has a loyal following and a bunch of haters. The "niche" that Bioware has is that they do story-based, character-driven RPGS that include romance content, with representation for women and LGBT players. They are really the only ones that are consistently doing this, although others are starting to see that it not only hasn't hurt their sales, but has brought in a loyal 'niche' market too. This particular niche market gets mocked a lot online because women and LGBT people get mocked a lot online period. It's nothing new. But just because Bioware has a niche market, doesn't mean that they aren't mainstream. I mean, Mass Effect and Dragon Age are hugely well known and well received series.
All that being said, I don't think that the fandom is ready for a trans romance. I hate that this is where my thoughts are, but it just seems clear to me that it wouldn't be well received. I think a transwoman, in particular, would really not go over well, even if she was an 'extra' romance (so that she didn't 'take away' from their romance numbers). I think that they should continue to add in trans characters throughout their games when they feel like it and, eventually, people will become more comfortable with them. Maybe in time, it won't be such a big deal, but for now, I just can't see it going over well. I think the first step should be a trans squad member or companion first.