I'm thinking it would be a nice idea if you could run for Jarl in one of the holds, which would be an upgraded form of the house-buying you can currently do. You'd obviously need a whole lot of money to do this, but this would enable you to perform certain actions within the hold such as raising and lowering taxes, being immune to the law, enabling the court wizard as a free trainer and other such things like replacing the destroyed buildings in Winterhold, establishing guild branches (for example, having a Companions base in Solitude), increasing or decreasing the guard presence in the various settlements in the hold (like you can at Riverwood), and funding expeditions into caves (you'd pay a group of soldiers to go into a dungeon or ruin, and after a few days they would return with whatever they found. The risk would be that they might not return with anything of value, or they could uncover a super-unique item). You'd need to be at least part-way into the civil war quests for this to work, because the hold would be 'given' to whichever side you're fighting for.
I think that's a fantastic idea personally, there are plenty of Jarls I'd like to replace, that Siddgeir fellow for instance. So a bit like an upgraded version of the system at work in the Shivering Isles
I could see it working, though I think it would work better at the end of the Civil war, if you replaced either Ulfric as Jarl of Windhelm, or Elisif as Jarl of Solitude (as she would then become High Queen)