If Ennio added to the soundtrack I'd rather it were something new to give the world its own atmosphere.
RDR had an amazing score which really set the game apart from anything else.
Indeed, a very minimal Morricone might work, in the same vein as the minimal soundtrack in RDR added atmosphere. I think he is too epic in scale to work in an interactive video game, however.
Morricone works beautifully with the films because of (and adding to) the epic drama. I think it would be a stretch to try to force this over missions or cutscenes, except for maybe one or two big dramatic transitions, like the opening into Mexico in RDR.
Slightly, related, I don't think the Tang Dream work added much to GTAV. It was probably originally conceived as a big scale art project like it would be used in Michael Mann's Heat, or the rich urban jazz in Taxi Driver, but again, the musical melodrama would overtake the game if overused. The minimal Tang Dream droning that was left in GTAV, didn't do much for me.