Quick question regarding "residual damage", wouldn't it be wise to roll dice to determine how long the effects last? I mean this is just me thinking but having it set where the player(s) determining themselves, one may say "[insert effect: eg burning on an enemy] lasts longer than need be, or, Oak Flesh[armor+] lasts 20 turns in a battle against a relativly weak enemy. 

Not saying I'd do something like that, but I have seen RPs where such has happened.

Aside from that, your idea sounds interesting