Make traderoutes that are done often less profitable and raise other ones over time. It eliminates the necessity for buffs (although they still can be used to show particular changes with the situation of bases and houses). If the players trade a lot, more and more traderoutes are saturated and you have to do the less profitable ones. Jump to a fighter for a while and the prices go back up eventually.
This was implemented to a degree on Hamburg City, albeit poorly. Could be improved here.