It's arguable whether or not Pray is PSI in nature or something else. That's a tangent, though, and I agree that new classes would be better than using D&D psionics, as well as adding some options for item-users like Lloyd, Jeff, and Duster. Delaying HP damage by a time proportional to the amount of damage taken could represent the scrolling HP in the games, Guts could be its own stat, or be subsumed by Constitution, crits are just a straight *2 to damage like SMASH, rather than rolling extra dice. That's all the extra rules that immediately come to mind, for me.