I hope modding support will be better than it used to be. Now, when guys from Rockstar have their own engine, they should be able to release some useful tools as well.
Lots of developers do this, and I'm sure that modding affects sales.
So, it's either gonna be harder or easier to mod the game. Probably easier, because we all understand how important it is.
Personally, I would love to see some kind of map editor and easier mission scripting (or some tool where you could script missions in 3D enviroment).