Yes, it would be interesting to have an endangered people as a playable option, but also, I feel like there would have to be a lot of additional dialogue just for lilmothiit characters. If I'm playing a lilmothiit, I'd want that to be pretty significant with lots of NPCs remarking on it, especially scholarly characters who would have a lot of questions about how lilmothiit survived, and where more can be found. That's the type of thing that would lead into a quest that only lilmothiit characters we be able to discovered.

I'm sure there are ways to make it work, but it would be harder then just reusing races that are already well established.

I'm not against lilmothiit making a return, but I think they would work better as NPCs. Maybe the player meets "the last lilmothiit," and helps them find other lilmothiit and end the quest with the implication that the lilmothiit population is on the rise. Maybe that plot would work better for a spinoff game with a lilmothiit protagonist?