Wealth. Don’t forget that by owning a stronghold, you also own all of the surrounding lands and impose a tax on all of the inhabitants. It will feel nice for a change to have someone recognize your high standing and give you the money that you so richly deserve.
I hope this is something which can't be exploited too easily, for example in the Fable series you could buy property and have the money rolling in within a few hours of play meaning questing and doing the mini-jobs for money wasn't necessary when going to the shop. Also the whole "leave it on overnight" could of easily been exploited (which I'm sure many people actually did).
I'm hoping that in very late game the party cap could be increased via stronghold upgrades for some extremely big quests/battles. Maybe use the whole party to fight the great ancient dragon?
Adventures for idle companions. You will eventually have more companions than will fit in your party, so you will have leave some of them behind. While they are idling away at the stronghold, they can take part in their own adventures, earning additional experience for themselves and extra money, items and reputation bonuses for you!
I do love this, I really hope to see this dynamically used so then there's a chance you see one of your companions fighting through a dungeon or shopping in the local market/town. Maybe even see the idle members pair up together for an adventure, this would make the game feel more in-depth.