The idea of markets certainly seems interesting. Perhaps something like this could work:
Duke decides to host a market and sets a % fee for all sales done within the market (and perhaps a flat fee to enter), and pays the necessary gold to set up all the stalls.
Nobles go to the market and decide how much gold they're willing to spend and how much of certain resources they want to take with them from their current caravans.
Trading happens automatically with nobles buying and selling resources (like the tournament rounds happen automatically).
A noble's chance to get good deals on buying/selling is based on a skill (do trader's have a class specific skill? Perhaps Oratory?)
This would mean it's a gamble when you go. Might come back with more gold, might come back with less (much like tournaments, except more 'winners', less 'losers'). The host may or may not gain gold (again, tournament style). And everyone gets to RP before and after the market about various things (preferably not badgers).
The only problem I see is the believability that all those traders will stick around for X days with gold and goods before anything actually happens. Maybe it could be that trading sessions are run every turn until the market closes? So the closer a noble is to the market, the more trading sessions you get.
As for the priest version, I can't come up with anything overly interesting. The only thing I can think of is holding a Conclave in a city with a large temple district. It costs thousands of gold to set one up (got to arrange appropriately sanctioned beds, food, activities, etc). Peasants hear about it and are converted in regions all over the continent (or within X miles of the city, where X is related to the amount of people entering the Conclave).
