The most impressive use of Once Upon a Time going forward is in Modern, where it helps decks like Four-Color Deaths Shadow, Infect, and Devoted Devastation find their key threats with frightening consistency
Khrn rewards us with card draw when he leaves our side of the battlefield, and we can mitigate any damage directed towards him by transferring control to a new player
The four main themes in the cycling mechanic are: Blinking and re-triggering enter-the-battlefield effects
Now we finally have some more all round playable sources of card advantage that a white deck can tap into
Importantly, the goal isnt to tear down your opponent (in Magic you normally started with 20 life the similarities are strong) but instead to race towards story enlightenment
Goblin Tunneler: limited needs this, it can also go well with a strong one or two drop that needs to deal combat damage to an opponent for a bonus