Rather than expand on cross-class abilities (which I think would end up blending classes together too much), I would love to see a complete revamp of the "attribute point" system into something similar to WildStar's Milestones or WoW's current talent incarnation. Those 30 attribute points are the useless feature, moreso than cross class abilities. Drop the system and replace it with a series of options that are all useful in different situations.