I think it would be hard to find a state of balancing it where it would feel acceptable at all levels of play. Its one of those abilities where the utility alone is so strong that it would always be strong in the right hands (re: Thresh, Blitz grab)
The biggest concern would be allowing players to reliably correct what should be lethal mistakes; getting caught out of position, ganked, or going up against a significantly stronger duelist. They can just warp back before the mistake.
There are things the could be done to balance it. There could be a delay between activating the ability and the "time warp". This meant that if the player is out of position and gets jumped on, they could still be killed in time before the player can warp out of it. This would be like a Reserve Tryn ult. With Tryn the trick is to not die for 6 seconds and then kill him when the ult ends. This way, the trick would be to kill him before X seconds so he can't get away.
Or, have it be something where it drops an item like a ward that you can destroy.
Player drops a Warp Beacon at their current location. For the next 10 seconds, the player can recast Warp to return to the Warp Beacon, assuming the same health and mana levels they had when Warp was cast. This does not reduce any Cooldowns of ability or items.
The Warp Beacon can be seen by enemy units and can be damaged by basic attacks. After 5 basic attacks, the Warp Beacon breaks, and Warp immediately goes on cooldown
That's just off the top of my head but its a way that you could have the same effect (cool time traveling ability) and if you plan it right can still be very powerful, but there is some kind of counterplay element to it, and reduces possible abuse.
There are plenty of other ways the ability could be done but that's just one example
