This is an awful idea, I can see how it might look good on paper, but it would end up either cheating the player out of the openness of the world, or or even worse negate most of the experience. The obvious example of a game that puts you on a timer is Majora's Mask, and for good reasons. The thing about Majora's Mask's Timer is that it does not really put a time constraint on your completion of the whole game, rather your completion of small segments of it. The reason this system works is that you are given the ability to turn back the clock at the very start of the game, and while turning back time negated some of the small progress you had made, the large progress was still intact. The reason this system works is that for the most part Majora's Mask is a linear game in the sense that you use items to get to new parts of the game, this way the game can indirectly point you in the right direction so you can make it in under your time limit. You don't need anything to go to the woods, but you do need arrows to go up the mountain, and you get arrows in the woods. To get to the beach you need a horse, but to get a horse you need the goron bombs from the mountain. And to get to the valley you need a hookshot that you get on the beach. The game has broken itself up into small sections that you have to do in order to progress, and while setting time back may set the post boss fight improvements back, you still have the equipment to tackle the next section of the game.
Now lets look at an open world game and why a timer, especially one who can be reset, would not be a good idea. First you need to establish a reason for the timer, in MM it was the moon crashing into Termina after three days,you can't just have a timer for no reason.The issue with this is that the addition of the timer will have a drastic impact on the tone of the game. The reason that Majora's Mask felt so different from OoT is because the effect the timer had on the mood of the game. A timer is a tool chiefly used to make the player feel helpless. I mean how were you supposed to save all of Termina in just three days? This would be okay for an a darker open world game, perhaps something with similar tone to Demon's Souls, because the game's tone would not be effected by this choice, but games like Oblivion (and Skyrim, though I have not played much of it, it looks to have the same general tone) would be vastly different games. The Elder Scrolls games have always been ones of fairly light tone, because at some point or another you would be strong enough to vanquish the great evil. Not only would it change the games tone, but most of the games mechanics as well. The fact of the matter is that an open world rpg's progression comes from a progression in avatar strength (i.e. your numbers get bigger but the combat is still in essence unchanged). If you put the game on any kind of strict time limit, there would have to be either a much more rapid change in avatar strength (most likely rapid enough to make it trivial) or more standard forms of progression would be used. For example a open world game on a timer might start you out with a class choice, that unlike most open world games, that had huge impact on the combat and progression and could not be changed. Let's face it when a huge hellspawn army is being built up you don't have the time to master all eight elements, become the best swordsman in the land, and become a tripple black belt you'd have to pick one and stick with it. The issue here is that this vastly limits the amount of content the player player can take part in. The inclusion of a in stone timer would vastly limit the mechanics ad content of the open world rpg. So let's say you go with a clock that can be turned back. A time limit that can be reset negates all of the non stat based progression a game can have. Say in the time you are given you save a town from a group of bandits on your way over to the ancient mine, well when you reset game back to day one to scale the northern mountain, those villagers are going to get robbed by the bandits now. One of the big draws of an open world game is seeing what you've done have an effect on the world, i.e. if you became the leader of a guild if you reset time you are no longer their leader. The reason that this doesn't ruin Majora's Mask is that you aren't trying to fix Termina, rather you are just trying to stop the skull kid, and therefore reseting the environments is not an issue as they are not the target of focus. I get where you are coming from with the idea of putting events on a timer in an open world game to add urgency to whatever your doing instead of having limitless time to just sidequest at the end, but it is simply not a feasible choice for a huge sprawling world, because players will want the time to explore it fully, and not letting the just counteracts the reason you made it an open world in the first place.
