To me, a classic JRPG is pure mechanism, a kind of puzzle. Was there some way of getting the fun out of building such a mechanism—of solving that puzzle—, wrapping it with the minimal amount of functionality, the simplest thing that could possibly pass as a video game?
It’s interesting that he went for speed. I’d probably go for a physical vs. magical split, to throw players on situational pinches - like, you built your char to be strong on a certain attack (physical or special = magical), but the monster is strong against that attack and weak to the other. Then set up the HP to a fixed amount (like 100), and increase/decrease damage based on the defences.
The result would be Atk/Def/MgA/MgD, so a bit more complex than his HP/Atk/Def.