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.
I'll agree that the current system isn't terrible. It's serviceable, but I believe it can be improved.
Honestly, a 20% damage difference IS very trivial in many situations, especially given the scope of other damage modifiers, especially if the trade off provides sufficient variety of skills to deal with multiple situations on a single load out. For example, if your extra memory slots give you +1 or +2 AP per turn, or Haste, or Rage, or Stealth etc, this can result in far more damage than just a straight 20% damage bonus from a stat.
Similarly, if you have a "support mage" who primarily just buffs the heck out of a specialized damage dealer, you get results that can sometimes surpass the damage potential of simply going a damage mage + damage character, depending on the situation.
My suggestions are to further enhance the system already in the game to further promote a variety of build options and to make Memory a much more viable stat that a single point increase in can make an impact.
