My official policy re: planeswalker spoilers is to not even try to predict how good they'll be, because it's pretty much impossible. Teferi is definitely interesting though. The (+) isn't card advantage, and card filtering has diminishing returns - at some point, you'll have the perfect hand, at which point more looting doesn't accomplish anything. It may be still be very strong if there are cards that benefit from being discarded or in the graveyard though - if you're running
Treasure Cruise or
Life from the Loam, it goes way up in value. I don't know if there are enough graveyard synergies in Standard though. If there are, then Teferi may see play there. He can also gain a lot of loyalty, since he effectively has a (+2).
Phasing something out is... also interesting. It's not really permanent removal - just a tempo play. Also somewhat unique in that it doesn't really protect Teferi if you use it on your own turn - you can clear out a blocker, but it will return on your opponent's upkeep. Instant speed interaction is certainly interesting though - you can temporarily shoot down a key combo piece, or just deal with an attacker.
The ultimate, at least, pretty much speaks for itself - two extra turns is strong.
In EDH / multiplayer, it's still somewhat difficult to reach the ultimate, but having a (+4) ability is sort of terrifying - that's a lot of loyalty to chew through. T1
Sol Ring, T2 Teferi into T4
Time Stretch is pretty backbreaking, although that says a bit more about Sol Ring into snowbally nonsense cards than it does about Teferi. The fast ultimate is definitely concerning though. But, again, phasing stuff out isn't amazing (although it does effectively last longer in multiplayer, since there are more turns the creature won't be available for), and you do still need some graveyard synergies to benefit from the looting.