re: Why aren’t there more open world superhero RPGs?

City of Heroes is still up and running on indy servers (look up Homecoming) and it still blows me away how well that game was done (not just for then, but now as well). The time and effort put into the content was top-notch. The only superhero MMO that touches it was Marvel Heroes and that was mainly due to the bajillion V/Os they had which would trigger randomly (giving a crowded area the more living feel as the character V/O's seemed to interact).

DCUO's main issues are relying on small but more frequent DLCs instead of massive, full-blown expansions as well as resistance to migrating from Unreal 3 to Unreal 4 (just the dungeon generator alone would save them shitloads of time in content creation).

Champions Online had some really great ideas but their game suffered (at least to me) from being so customizable as to be overly complicated. I never could wrap my head around their crafting system and how it impacted powers (at least not enough to feel competent in what I was doing).

If such a game as you describe came out, it would ideally be a voxel-based world, something along the lines of what they planned for EQ: Next (also: Landmark) so players could really have some fun with not just the standard powers but ones which would normally be game-breaking like phasing or growth/shrinking.