What's a card that, when it came out, felt so obvious in hindsight that you're surprised it didn't exist sooner?
For me, it's Ready For A Fight. A very simple yet game changing card that gives Protection easy access to alter-ego in a way that feels very "Protection." Like this is something Protection should have had all along instead of 6 years into the game's life.
Another, also Protection, is Stun Gun. With Stun Gun, it's not that the design itself feels obvious, but that it completes this cycle of Tech Weapons that started back in Galaxy's Most Wanted. You have Hand Cannon, then Sonic Rifle, then just nothing for several years until finally they drop Stun Gun.