What I would love to see is for the 'no lands = game over' mechanic to be removed, and a whole bunch of events / plots / decisions / ambitions being added to give enough gameplay to non-landed roles that it's possible to play as a courtier and try and become a landed dynasty. Or a baron/mayor for that matter.
This came from a conversation with my mate that feels you should be able to start as a custom dynasty, and I said I felt that giving lands to a non-historic dynasty would fly in the face of the historically accurate start dates.
How much UI modding is possible in Paradox games? Would be cool, though I'm not sure / doubtful if it's possible, to be able to start as an unlanded courtier in any county, and type your own dynasty name in. You'd have a whole bunch of events associated with the various jobs, chancellor, spymaster and such to keep the player busy (having to make decisions in events relating to your assassination missions as spy master, or trying to fabricate claims as chancellor), and other events / decisions relating to every day courtier life, effectively being Blackadder in Blackadder the Second, trying to curry favour with the ruler and engineer the situation where they could rise in status, in the meantime they would still be getting married, raising children and keeping their dynasty alive all the while trying to get a count, duke or king to give them a landed title.
Obviously this could only go so far as a mod, I guess beyond making events / plots / characters and other things I've spied in the data files I don't have much knowledge as to the moddability of the engine. I know the game uses lua but how much, if any, is exposed to modders? I've not seen any game mechanic code anywhere and it seems to mainly be data.