Sooo... what kind of running-around-inside-the-TARDIS strory is this going to be?
1) "The Invasion of Time" - wandering round and round and getting lost
2) "The Enemy Within" (not the one that was filmed) - the Doctor goes to confront the great evil at the heart of the TARDIS
3) "Heart of TARDIS" - deep in the TARDIS there is access to the nebulous realms from whcih you can get back to the TARDIS but in the past, and so potentially meet past Doctors
4) One of Tom Baker's ideas - all the old Doctors still live in the bowels of the TARDIS ins ome way,shape or form.