Here are a few updates I've made to my TARDIS since I posted this thread:
A lift to take you down to the lower floor
Some additions to the lower floor to make it a little more homely (armchairs, food machine, jukebox, gramophone, chest, safe, crafting table, furnace and some lights in the floor)
A wardrobe (except it's the size of a room)
A library / swimming pool
A bedroom (it's a bit empty except for the bed but oh well)
I also made it so that if you break the TARDIS doors you can see them opened out into space, giving the effect that you have opened the doors during flight.
Feedback would be appreciated (if anybody happens to read this thread XD).