I don't think merging more than one zelda game together is something Nintendo would ever do. And i really don't like the idea. But I do like the idea of time travel, if used in a good way. And depending on where the new game is placed, you probably wouldn't be able to reach the worlds of PH or ST. It would completely demolish any timeline ever created if you asked me.
Maybe time travel could be used as part of the gameplay. Like say you messed up somewhere, and you had to go back in time to redo it. Kind of like Prince of Persia.
But it seems like every way you could use time travel, it wouldn't make sense to use in a zelda game. It would take a lot of programming to do something, then go back in time and see yourself do that event again. Ah i don't know i'm just rambling lol
But the idea of time travel is awesome in my opinon. Time travel is one of the rumors that have spawned over the past half year, so who knows.