How about not add anything new because the base system is generic enough that it allows you to specialize or 'cross-class'.
Then again we aren't playing the same game; in your world 20%+ damage difference is a trivial amount, and a character that strikes for 10% less damage but has 9-10 more memory slots is insufficient. Hybrid characters do exist already -- when you can competently deal both physical and magical damage you're there. You just haven't pushed the system to its limits.
