I would define character classes based on the books, not RPGs (neither Stormbringer or D&D - in fact that would be a general design concept to come back to Moorcock's fiction instead of trying to stick to a role-playing style). This would include capacities with player choices and versatility a la MCAU/E.
I would redesign the magic system to evoke the books (and use probably Call of Cthulhu d20 as a base concept for a d20 DLoM magic system).
I would break down the feats and implement regional feat selections and feats depending on your ascendance (Melnibonean, Pan Tangian etc) and allegiance (Law, Chaos, Undecided, Grey lords for a supplement).
I would make it a playable role playing game on its own but still compatible with D&D (for instance, one would be able to take rules of allegiances, boons and curses and summoning rules to use them in D&D instead of the summoning spells in PHB).
I have more ideas about the subject, but I won't bore you with the specifics.