If disbanding troops is permitted, I would suggest that troops disbanded outside their home province should at least sometimes be converted to unrest.
(Perhaps even if -in- their home? And troops that desert due to low pay, should almost always cause unrest if they don't already. Brigandage and all that, you know.) Along similar lines, it might be interesting if mercenary bands despairing in their unemployment would look for weak provinces to plunder... or perhaps be "hired" by an independent province, even.
If I were being really cynical, I might suggest that spies be allowed to attempt to bribe other nations' mercenaries even the latter's contracts -aren't- going to end the next turn, although it shouldn't be feasible to be able to bribe them into a certain-death situation (sneak away into the bribing side's province next door, perhaps; try to take over the massively garrisoned capital province with nowhere to retreat to afterwards, no. They're in it for the money, not the glory of death in battle.
And, being mercenaries, it wouldn't be out of character to take the bribe and betray the one who offered it.)
Continuing in the line of deceit and treachery, I wonder if experienced assassins should be permitted the initiative in battle (not when they're caught, that is).
There's a lot that potentially would be amusing in terms of counterintelligence (allocating spies to disinformation so that enemy scouts and spies perhaps get bogus information, say) but this would be rather hard to do well and realistically methinks. If a scout could be caught and turned so that, for instance, he'd keep reporting whatever he saw, troop-wise, the view from the turn before he was caught, then it'd be possible to hide the movements of an army...
A wargamer might wonder if there's a use for a force-marching option (e.g. get an extra movement point for the ground-pounders, at the cost of -2 or so morale (or more, even) and heavy fatigue (40-60+ ?) upon their arrival, and a penalty with regards to how much supply they're getting from provinces due to spending less time on that (which means: bring a supply train, er, broth etc). Bad if you're going to attack immediately afterwards, but the potential surprise factor may help and it might save your bacon if you find a reason to shift troops in a hurry.
On mobility, given that we have different movement classes (water, land, air) it might be interesting if a .map file could specify limitations on adjacency (e.g. province 1 is adjacent to province 2 for flying units only) so that one could specify rivers that could be flown or swum across, or perhaps a mountain range between two provinces that's an obstacle to flight, but has mines / tunnels that offer land units a way through.