I hate to be that guy, but I don't really see the need when Shadow of the Demon Lord is a d20 game designed as Rob Schwalb's "what if?" edition of 5e D&D / Ode to WHFRP that covers almost all the same ground as what I would imagine or want from a d20-based Diablo TTRPG.
Interested, but I don't just want the setting ported to 5e rules. I want Diablo style item generators with affixes, and I want powers that use mana, or have round-based cool down effects. I don't just want reskinned 5e.
It should use the same basic rules framework as 5e. Just dump the spell slot paradigm, daily powers, and the entire concept of rest as a recharge mechanic. It should play like a tabletop version of an action RPG. The dungeon diving shouldn't stop because you are out of spells. That shouldn't even be a thing.
And don't be afraid to give martial characters similar abilities. Give martials more than just here is some bonus dice, or you get a couple of extra attacks.