I appreciate the wall of text! I may do another double door first, but I feel like the more specific loot should be on the higher tier garage doors. After that I can always do some extra double doors for more misc. category type doors. (If any more get accepted at all, who knows )

I don't really like having an upcoming skins image tbh, I prefer just seeing what happens, no clear plan. I will update the collection header with all the released skins though

[sorry for walls of text]
My guess is that you can look at what neon storage/science storage/graffiti storage skins sold well to get an idea of what sort of loot rooms people might like. An armory room and different types of mechanics rooms would make a lot of sense, but I'd have to analyze people's bases to see whether people prefer garage doors over double doors for those.

Glowing garage doors also sell really well, but there haven't been that many double doors skins, so they will most likely sell even better. Considering facepunch is probably very willing to give you an entire set of door skins, it might worth it to make a bit of a map like they did with graffiti storages, that showcased all the released skins, and a bunch of blacked out storages with question marks put on top of them (I could see if someone could do this for you). This may get a lot of people anticipating a proper complete set for doors which hasn't been done before. If on top of that you made a lot of (desireable) double doors, and a couple of desired garage doors (all of which glowing or not), your entire set could be as popular as the neon storage and the graffiti storage collection (which sold A LOT).

Thanks! :) I might actually try some loot-specific garage doors next

Big fan of the set. By the way little word of advice as an investor, double doors tend to sell better, which is good to keep in mind if you plan on continuing the set. A lounge room wouldn't be too bad of an addition I think.