I've been thinking of interior ideas for a while (I've been thinking about trying to make them myself, but I've never worked on a single thing related to Unity in my entire life, so there'll be a learning curve)
I really liked the idea of a mall interior, I was surprised to see one had never been made yet, considering they were one of the main locations used in a lot of liminal space projects and art (The Rolling Giant). I envisioned it like one or a few large atriums with non-functional escalators/stairs with most of the scrap contained in the stores. Another idea might be one with no atrium, but long stretching hallways lined with stores.
Another idea I liked was the idea of a parking structure interior. It sounds strange, but if you've ever played the mall parking map that blew up on Gmod during the nextbot fad that's basically what I'm picturing. I imagine it more vertical than most interiors (since parking structures are layered) with large, open spaces containing derelict vehicles (or maybe none at all) and various maintenance rooms/tunnels, small offices, security rooms, etc. on the sides and within hallways. I'm not entirely sure how it would play with Lethal Company's generally cramped interiors and AI designed to act within them, but I think it would at least look good.
This last one is probably the weakest, but I was also surprised to see no one (technically) had really made a hotel/apartment themed interior. I know that the Gray Apartments interior exists, and I do use it, but it's just their OneShot interior reskinned from purple to off-white, it feels off to me. I think something like a hotel would work well as a counterpart to the mansion, following a similar design, but I don't have any great ideas on what this would look like or how it could stand out.
(attached pictures are the aformentioned mall parking map and an old zombie survival mall map from Gmod)