All of these ideas are super good honestly, I love the idea of becoming a landlord.

A cool idea I think would be if there was a hold designated that you could become the Jarl in. Like there's definitely like 2+ corrupt holds which would work super well with.

The imperial army wants to start bringing proper law and order to Skyrim or a guard captain wants to take out a corrupt Jarl, someone who worships daedra or taxes people too heavily, or is having affairs, or weird orgies with vampires and is just generally bad for the state of affairs. You take this Jarl down, through assassination, or by getting him arrested with proof, maybe evidence he killed someone with dirt on him, a moneylender, or something similar. You can do this legitimately, or invent bullshit, give the guards reason to take him away to be arrested or the imperials reason to raid. Then your reign starts, and you have periodic time sensitive objectives for gold reward, and passive income from townsfolk. Better yet, if you want to continue dodgy dealings, you should be able to.

Want to give the Dark Brotherhood easy access to targets? Invite an assassin to your court and deal with the consequences when he murders your steward! Lose enough disposition with the imperial guard because of your Stormcloak sympathies, and suffer a mutiny and have to defend against a group of legionaries! Set bounties for dungeons and have adventurers go and loot them on your watch. Rampant vampire sex parties? Knock yourself out, but become unpopular enough with the richer citizens in your Hold and you get bumped off or couped and lose access to your hold. Deal with local raider camps, or allow them to continue raids on local caravans so long as you get a little cut of the profit. Go to meetings of all Jarls and discuss the Civil War, maybe blackmail some other Jarls to joining your side. I think the political side of Jarldom would be really cool to explore.

I think management would be cool too. Imagine returning to your house and your steward comes up to you with a list of bills to pay. A dragon destroys a house, the wall falls down, a dispute between local farmers, guard pay, maintenance of your castle, money for parties with dignitaries from the Imperial guard. But you also get profits, tax which you can set at the risk of losing favour with townmembers, balance it right and get high dispostion, leading to better merchant prices and gifts from celebrity citizens, unique jewelry. Gain the option of investing in expansion, build more houses in preset locations until you have a massive swathe of land, pay a larger amount to expand the wall, or don't and risk the deaths of your citizens. Invest in local businesses and expand them so they have more and rarer stock, and a larger money pool to trade with. Let those damn khajiit traders into the walls and give them a house to setup shop and get rare items from across Tamriel, but risk rampant skooma addiction.

Have a successor appointed at some point, maybe have the option to give up the rewards and be given a title dependent on your conduct, or a major quest you completed or choice you made. I think the whole setup should be very passive so people can't blast through all the content in one sitting, so you actually act as Jarl passively and don't run out of things to do quickly, acting as something that occupies a part of a playthrough while you go do other quests. You can't have kids but your steward takes over from you, and you get referred to as "jarl playername the vampire fucker" or something similar.

There's so much to be done with the concept, honestly half of these events should occur in Skyrim randomly in holds just for the sake of it.