(As I explain this, I do realize both genders play this game, but for simplicity of explaining my ideas, I will reference the ideas from the male perspective to keep it simple and clear as possible.SHM)
Everyone knows it is important to keep one's wife in PG happy. If you do, they give you a voodoo card. The happier she is, the more frequently she is generous. If you have a mistress, they make you more famous, which is good. You can spend many turns to do this. What if a mistress could provide you with "an opportunity" to profit?
Mistresses in this game would be privy to much information, scuttlebutt and rumors from all over Avonmora. Perhaps of a merchant in Akrotiri who is selling products for half price, or one in Aiora who is buying at double the price. It is a limited opportunity, say only 10,000 units, or for one day. Perhaps she has heard of an abandoned warehouse loaded with goods that if one had the resources, could take and sell in another port.
Perhaps she has heard rumors of an abandoned ship that is just setting in some port, on an derelict dock. A story told her by her aunt about buried gold in Pania.
....or, even more useful, one's mistress could have reports of another player's trade route - a single route - which would provide one opportunity for plunder. Perhaps she even might know something about other player's trade routes. She would reveal the trade route of one trade fleet from a random player. Once revealed by his mistress, the player would have until the end of the day to attempt to plunder that one trade fleet. He could move his battle fleet to the port, hit a "raid" button, and it would add 3 DR points to the trade fleet - if it was there. It may have been a false rumor, and she may have just told you something to make you happy!
This would add a feature to why one would wish to have a mistress, besides for simply fame. At best, each rumor could have a 50% chance of being true, if her happiness is 100%, and she's walking on clouds. The opportunity would only good for 24 hours, or until the end of the day turn.
I am sure there are other ideas that a "gossip gatherer" could provide; providing both NPC and PvP opportunities. With 20 mistresses throughout Avonmora, there would be several opportunities for players to "engage" each other simply based upon what the information one's mistresses provide.