I've spent the better part of the day watching and reading about the GM mode feature, and I really like what I see so far!
One thing I thought of though is that it would be awesome if there were some kind of tools for dynamically adding new things to character sheets. And by this, I don't mean pre-created content like skills or inventory items, instead I'm talking about things that exist outside of the Divinity system but would still have significance in the GM's game.
One example would be if the GM granted a player some kind of custom ability that they could use once per session and would be resolved by dice rolling, maybe like an energy drain spell that causes the monster to lose two levels if they fail a save.
Another example would be if the GM wanted to include some kind of (non-combat) skill system that would essentially just keep track of points players have in the GM's invented skills (or skills from an existing PnP system). I noticed that most of the checks the players rolled in the demo sessions were just ability checks, there didn't seem to be any easy way to keep track of skills like persuade, intimidate, etc. It would be great if we could modify the character sheet to just keep track of these.