On the subject of timed missions and restrictive missions in general, and even more generally on the subject of "failure" I always thought City of Heroes players were a little too spoiled in terms of expecting success, and wildly successful success in particular as the expected norm, due in no small part to the limits of CoH mission technology. Nearly every core XP mission was basically a skinned combat environment of the kill or be killed mission. And no one wants to be "be-killed."
But I think it would track better with the comic book world in general if there was a little more grey between "dropped dead" and "genocide." For example, I think it would have been legitimate to make a timed mission in which, say, you had to fight your way to rescue a hostage, with the possibility of failing the mission *if* that failure wasn't a binary failure, but just a variable in the mission arc. If you succeed in saving him, maybe in the next mission it turns out he was a traitor and you have to fight him. Conversely if you fail to save him, the mission arc goes off on a different tangent instead with different mission content.
Success or failure should, in general, I think be thought of in terms of overall mission arc goals, not in terms of individual fights, or spawns, or even instanced missions. You can satisfy both the power builder power gamer and the more casual player by constructing missions in which the power gamer blasts through the mission largely on firepower, while the more casual player struggles along a different path that ultimately generates comparable amounts of character progression and an equally rewarding story.
I think its okay to fail, and for the game to create the circumstances in which many players fail, so long as failure isn't ultimate failure. As in comic books, the game environment should be one in which death is not final, failure is not final, and there are always second chances. True, final, permanent, non-repeatable failure should be extremely rare. Temporary, repeatable, bypassable, ultimately surmountable failure is something that I think CoT could use more of, relative to CoH. I think it just makes running missions more interesting.
One thing I thought was the exact opposite of what makes sense for inclusive casual play was CO's original death penalty. Every time you died, you got a temporary self-debuff. You got weaker. Which means if you died in a tough mission, good luck trying again. After three deaths without the penalty expiring, good luck not getting killed by random spawns just trying to cross the street. After five stacked deaths (the limit if I recall correctly) good luck not dying tripping off the sidewalk.
That's backwards. Sure, you're rewarding good play by penalizing bad play, but conversely you're also punishing weakness: if you make a mistake, the game makes you more likely to make another, and even more likely to make another, until you have no choice but to walk away from the game and wait for the death penalty to expire. But in a game that is trying to help the player to succeed, you'd want to do the opposite. So if you die running mission A, maybe the game tosses you a bone and sends the City SWAT team in with you on the next mission to help you out. If you blast through the missions without breaking a sweat, maybe the next mission is just a little harder.
In the same way I think failure should be an option, I believe City of Titans should be like Hogwarts: Help will always be provided to those who ask for it.