So, i'm not really a map builder, but I thought I'd give it a go. I was thinking like, you start by stealing a tt capsule, and you land on earth. you then earn intelligence from doing different tasks, and you unlock different planets and better tardises. The ultimate goal is to stop all evil in the universe. The tasks could be stopping those evils.

Any thoughts/ ideas? 

you should make a custom tardis interior for the map

I will help u if u want

maybe you could put crashed tardises and bosses in them? maybe some references to old episodes?