I think that you should be able to make and add rooms to the Tardis but it costs lots of materials, like zeiton, or something else.

 If you wish to suggest specific rooms I recommend seeing Milk's thread TARDIS Interior Suggestion Thread (Official)

If you're suggesting a method for a user to create rooms for a Tardis (other than block by block and by hand) I recommend adding many more details.

As suggestions have already been made on the topic of "room creation," this thread serves no value unless you have a unique method of implementation in mind.

I recommend thinking about the following:

• Think about how the player will choose to place rooms
• What will the player use to create rooms and place them.
• How will players be prevented room placing rooms on other players' Tardises
• What happens if a user accidentally winds up overwriting another part of his or her Tardis and will the player have a way of knowing if the room will line up with a pre-existing corridor on placement?
• What will be the cost?

 ...and then incorporating these ideas into the thread.

It would make the thread much more interesting and add some substance, although that's just my advice