What would really make this site much better for me to keep all my materials of the game in one place would be the ability to put my game notes on the site. Much like the handouts section but only ever for GM eyes. Currently I use google docs for my session notes but having it on the site I would find to be much easier. When opened it pops up in a new window that I can still interact with the main site.
Also another suggestion would be something like a forum for a create game. A place for character sheets to be stored and other things like side missions that can be done almost play by post style in between game missions.
If either of those two options were put into the site would make me pay for the next level of membership.