If one were to have time travel (backwards in time), I think you would have to carefully consider how memory would work.
I'm thinking that if past events cause present memories and motivations, then if the PC's go and change the past, they will change their memories and it won't be clear why they decided to go into the past. In other words, they would have to remember something that would have happened but didn't really, and then act to ensure that it still doesn't happen.
Their actions in the past cannot eliminate themselves or their ability to travel in time. I see them as being compelled to follow a script that leave the time-line substantially unchanged, but perhaps with a few little quirks. Remember in "Back to the Future" where the "Twin Pines" mall changes to "Lone Pine" mall because of Michael J. Fox's actions?
Maybe if they try to depart too far from the "script" everything turns all hazy and wonky- meanwhile all the denizens of that time are reacting to what the character is supposed to have been doing.
So I would probably have at least two feats dealing with memory; "prophetic memory" (to remember a possible future) and "paradoxical memory" (to remember an alternate history).
If characters have access to paradoxical memory they will know why they need to do what they need to do, and so paradoxes are avoided. If they have a prophetic memory they will know what minimal actions to take to nudge the time-line in the right direction. I imagine that a common action will be to hide some important item in the past so that it can be retrieved in the present. Or supply a motivation for a key NPC to take an action in the present that he or she otherwise would not have done. Like, say you need to befriend a guard now- go back in time and make friends with him at a more opportune time.
Just my two cents.