Single-player make your own hero game.
IF it's good, sign me up.
There are two ways to make this good.
1.) Freedom to create a character who functions similarly to any comic book superhero you can imagine. This would require them to make a range of AI types, quest systems, behaviour patterns, etc. to cater for very different types of characters with very different types of powers and methods of finding and stopping crime.
2.) A game built in a universe with some explanation for a source of superpowers which are in the hands of formerly normal people (Mutants like X-Men popping up, biotech, or some other explanation would work). You could then create a character by selecting from a predefined list of powers, and either upgrade those powers or add new ones as the game progresses. This would allow the developers to construct a cohesive world a lot easier, because they could limit the number of approaches the player can take to different situations, thus enabling them to stick to an established gameplay system without sacrificing balance. To do it justice, though, you'd need either a game-specific alternate timeline to an existing IP, or a new IP to do it with.
Option 1 is the one which sounds most like what the OP wanted, but it WOULD take more work than a lot of you are suggesting. Look at good superhero games, and how they play.
Pretty much narrowed to "third-person" of some variety, but they range from in-depth stealth (Arkham Asylum), to blatant power-trip hack 'n' slash (Prototype), to cover-based shooters (Infamous). And that's only three of the better known ones released recently.
Look at the enemy types, there's very little similarity between their AI behaviour, both in and out of combat.
Stealth games require enemies to be slow to respond when you're sneaking around, or the game loses tension through repeated death or repeated failure to die when found, cover-based shooters and hack 'n' slash games require them to be fast to respond, or the game feels too easy. Ranged enemy attacks when you're a melee fighter need to be powerful in stealth gameplay, but weak in hack 'n' slash gameplay, and balanced with your own when you're a ranged fighter yourself. Similar things need to be tweaked for melee-focused enemies too.
Does that answer the question about why an open-world superhero game with custom characters would be more difficult to make than other open-world games?