It would be great to develop castle, skins potencial far more. How? My suggestions:
In-game economy (could helps in many ways to obtain skins or some nice addons) that can be obtained on champion/legendary difficulties. It may result more players going on harder stages.
Unlockable areas in castle for in-game cash like additional rooms, basements, towers that could be modified with in-game money
Items like furnitures, banners, statues you could place in heroes quaters or unlocked areas
I think it would be nice to see new areas unlocked in stronghold, and even more nice to see them customised by myself
It is really fast idea, I didn’t come up with solutions because it will surely take time and hard work to do, but you can also make faster way for those who are willing to spend money for that by selling big packs of this in-game currency for real money, but have in mind somebody want to earn this things by playing.