I've been pokeing around with the SLUDGE
engine for making adventure games. Infact I used Doctor Who as a test setting. Unfortunatly I can't draw worth beans.
As I'll illustrate here...
Yeah.... you should see how badly the Doctor looks when he walks.
A Terra adventure game could be very very interesting, it does have the issue of multiple main characters, and the need for some tweeks for puzzles.
Thoughts occur, you could break the game in chapters, in each chapter a different main character which is played.