Liliana, The Last Hope killed off a standard archetype with one card(White Weenie died like dust in the wind, because Lili straight up kills the Critical mass of 1 Drops)
Gideon, Ally of Zendikar is a forced card to play in any Deck that is White right now, because he's so stupid on a cursory glance alone.
Basically, of the Gatewatch, the only ones not to see a format defining card yet is Chandra, Nissa, and Ajani. Nissa Vital Force would be there if Modern wanted her, because turning a fetch into Draw 2 is all kinds of dumb.
You early Rotate Gideon and half of the vehicle problems get fixed, because then there's an actual cost to Crewing Heart of Kiran(Either 3 Power or Loyalty counter from Planeswalkers whose ults say "you win the game", meaning the Crew Cost is, Shocker, an actual Cost). Other vehicles get worse, because the number of cheap Crewers goes down.