Use skyboxes for building interiors!
Turn doors and other entrances into teleporters so people can enter and exit. This frees you up from the constraints of the environment and also makes sims less laggy by moving a lot of content out of rendering distance!
For example, if you have a house at ground level, make the door into a teleporter that takes people to a skybox 400-500m up! You'll see a huge FPS increase doing this because your computer will be rendering far less at once. And since you can build up to about 4000m up, you can space out your interior skyboxes. So if you have an enormous interior area you can split it into sections, spaced out to keep framerates high.
And if you use an Experience key on your land, you can make it so people can just walk into the doors and be instantly teleported to the skybox, for a relatively seamless experience.
If you have limited prim space you can also use this trick with a holodeck style rezzer system so building interiors are only rezzed when you're using them! This way you have virtually infinite prim space.