This is something we tried in our game last week to get more wandering monsters, and restrict the length of the game without an arbitary turn limit.
At the end of each turn, each player rolls a dice and adds the turn number plus the total number of treasures claimed.
18-20 - level 1 encounter
21-23 - level 2 encounter
24-25 - level 3 encounter
26-29 - ice storm brewing. Next roll of 26+ by either player ends the game.
30+ - game ends immediately.
If a player rolls an encounter, he rolls for what type of creature appears, then places it himself - it can be placed on any table edge at least 3" from any figure, or anywhere on the board, out of LOS of any figure and at least 6" from any figure.
It worked well. We probably got a few too many creatures on the table by the end of the game, but then we were playing the Living Museum so we also had animated statues involved.
When I get round to re-writing the encounter tables to reflect my figure collection, I will probably replace some of the creatures with events - normally these will be beneficial to the player that rolled them, for example extra treasure token found, extra actions for figures, free move of creature, or random enemy figure loses next action.
This system could also incorporate a balancing device for different level wizards. If the enemy wizard is higher level than yours, you may add a number to the dice roll (maybe half the difference in levels). You could choose to add up to this number after rolling. So a warband led by a higher level wizard will find itself facing lots of extra wandering monsters. This makes sense thematically as they are attracted by the fame and wealth of the wizard.