Probably a shooter that is less linear trench-slogging with quicktime-events that were thrown-in to make sure the buttons you normally don't press still work...

and more... well..

I was thinking something that was more "final objective oriented with no care how you get there or how badly you screw-up in the process so long as you can fix it" with several random events set into motion that could totally force you to fail and need to be expertly manipulated without any hand-holding to line-up with the final climatic showdown...

Kinda like trying to manage kharma in Fallout to get good/bad endings, but instead it will constantly change to something you don't want it to be without constant upkeep, like letting your reputation fade into obscurity and your deeds suffer from constant entropy (and having some missions that REQUIRE you to lie-low, to LET your kingdom crumble around you in order to undo a simple mistake, to act in secret).

That tiny little realization that even though your deeds will be your legacy, that legacy can and likely will begin to fade the very second it fizzles out. So you act. You move. You give your world something to talk about.

You also create your own difficulty spikes by how you prioritize your mission-objectives, path-finding ability (Portal/Mirror's edge?), and time-management to deal with random events that could fatally bog your character down if you take too many risks too often (leading to players horrified of the idea of running behind their own pace).

Example: it could be possible for any NPC to forget who your character is (without scripting), which may or may not be a good thing. Especially if you're on a time-sensitive mission that required their cooperation. Sure, you could earn and hold onto a keepsake to remind that NPC you've earned their trust (or the trust of another NPC they trust) but that takes up inventory and itself will be fragile.