I've been messing around with this Mod for a few hours, and am planning on setting up a private server to use with friends, so far this is incredible. It'll definitely be a lot of fun. There are a few things I'd hope can be made possible:

1. Option to either have each player have their own TARDIS world, or modify the boundary size in the TARDIS world (for instance, set it to a large size such as 5,000 x 5000).

2. Hallway generation by importing your own custom designs. It'd be nice if you could have rooms bridge together automatically by right clicking them with a hallway design of your own.

3. Battle TARDIS functionality to go to war with other players. It would be a fun way to roleplay, just jam their controls, shoot at each other to cause damage to interiors.

4. Importing your own TARDIS exterior designs (if this functionality already exists my bad). I had this custom model I'd made in a Minecraft world and exported for an animated project, it'd be great if I were able to import it into the mod.

Apart from that, everything else is great. I'm already planning out my own horseracing arena and garden.

The first 3 are definitely more of a dev thing, but as for #4 you can do this with resource packs and replacing existing exteriors

This video has a tutorial and download link to the MDL converter so you can make an OBJ model into an MDL that can
^ link                  replace an exterior