I suspect the generator rules can vary with time, but to fend off chaos they should be a deterministic function of the content of previous block(s) and not depend on the transaction pool. I also suspect that if the customers & merchants do not hold a significant fraction of the Ghash/sec, the 'rules' will tend to make a generator cartel with high fees.
Yes I believe the real potential issue is the 'cartel,' I'm not worried about the transaction fees being too low, rather too high. I think that studying this with game theory would be very fun, but, I don't have the time also.