I like the idea of a cooperative-yet-competitive relationship with the other groups living on the turtle. You all need to work together to feed the turtle, otherwise you all suffer. However, maybe one group will benifit more from having the turtle pass through a rainstorm, but it'll cause flooding and/or crop failures for another group. So there's some competitiveness in steering the turtle. Or maybe one group benifits by having the turtle languish in a sunny spot, making it go slow by nearly starving it, but another group needs wind for their windmills and wants the turtle to move quickly.

Regional resources could get depleted if the turtle hangs out in one area too long, or maybe pollution or something builds up, so that encourages players to want the turtle to move around more and not revisit the same areas very frequently.

Finally (and into serious feature-creep mode) what happens when the turtle encounters hazards? Creatures that attack the turtle or dispatch "parasites" that attack the people living on its back? What if you encounter another turtle with its own host of civilizations? Will there be conflict or cooperation? ...so many possibilities!