If an effect allows opponents to cast a spell as though it had flash (for example, if your opponent also controls a Teferi, Time Raveler and activates his +1 loyalty ability), the restriction of Teferi's first ability takes precedence over that permission.

You may activate Teferi's last ability without choosing any target. You'll just draw a card. However, if you do choose a target and the target permanent is an illegal target by the time Teferi's last ability tries to resolve, the ability doesn't resolve. You don't draw a card.