NOTE: This is just my personal opinion, you don't have to listen, I'm just sharing my opinion
DOS:
Base levels off of Liminal Spaces, usually hallways or doors or things like that. To me, those feel the most backrooms-esque. Also, It might just be me, but I feel those ones are the most well-written.
Make entities FEEL like they should belong in that level. It makes sense for partygoers to live in a place like Level Fun. It would NOT make sense for them to live in Level 2.
DONT'S:
Make "All powerful Backrooms gods that want to destroy everything and all the entities fear them"(looking at you, Redacted Watcher). Those tend to just feel cringy and unfitting. Like, the Backrooms are meant to feel creepy and unsettling. Making a powerful god-like creature with power like that of the Doom Slayer just throws all that out of the window, and makes it feel like a poorly written superhero film.
Make entity hordes out of levels. We have Level !, as well as Level -150 !. That's it. Only exception is a scenario like Level B1.
Make city levels. We have enough. City levels in my opinion don't really seem to suit the Backrooms. For now, let's just call it a 1 and done thing.
Well, that was my list of Dos and Don'ts. You can add suggestions in the replies.