[Tyranny was niche though, as you mentioned, and I think that a Warhammer/40k is not the same type of game or experience that a single-player, story based RPG like BG3 is. I'm not arguing that there's not a market for it. I am arguing that BG3 won't be going down that path. Does anyone really think that they would?
Well, BG3 is in the world of Faerun, which compared to warhammer fantasy is a teddybear realm. But Larian stated it will be much darker than DOS2, which would entail they are exploring all possible paths in the alignment system. That would be my guess.
Suffice to say, you could play an evil character in both BG1 and BG2. You were a bhaal spawn after all. And let's not forget that in BG3, you have a tadpole in your brain, and it's not really friendly either.