For a while now, we've had the Galaxy class model from Earth Spacedock's Ship Requisitions, and the B'rel from the re-mastered Doomsday Machine mission. At one point, we also had ships that orbited around the Earth Spacedock interior map - a Luna class, for example.
I think we could make some very, very interesting missions if these assets were added into the Foundry. For example - imagine having a First Contact-esque fight on the exterior of a Galaxy class, or walking along the nacelles to fix a plasma leak. Both of these ideas could easily be accomplished with the current Foundry tools if we had the ship models to work with.
I'd love to hear other people's ideas about missions involving the exterior of a ship, and how it could be done. Seeing as how the models are already there, I can't really see there being a problem in making this happen.