How about banning slavery or greatly restricting it? Possibly caused by a major slave revolt?
This would greatly cut down the amount of money the generals could get by conquest and enslaving the losers which would make them less independent of Rome.
It might also slow down the rapid expansion of the empire because if there is less wealth to be found in slaves there is less reason to conquer and more reason to develop lands they already have with Roman freeman.
It would also stop the flooding of Rome with slaves which drove lower class Romans from their farms and other jobs and prevent them from becoming dependent on politicians and generals who promised free food.
Not even a slave revolt like Spartacus could end slavery in Rome...