Many dungeons or full campaigns that I'd be interested in making require a stronger handle on the classes and races that are allowed to play. This may be a matter of balance or character power regulation, or it might be part of the narrative in the game (a non-magic world that allows wizard characters would be a problem).
I'd like the ability to create classes and races from existing assets. I'd also like the ability to allow or prohibit feats/abilities within particular modules. Potentially, if it isn't too burdensome, the ability to create or modify feats and abilities would be great as well.
Of course, for any or all of these changes, the module would need to be able to recognize characters created within that module and accept only those that belong.