Teferi's Protection A Game Changer in Commander, Teferi's Protection is the pinnacle of protection spells, saving you from death, protecting your permanents, and, in some cases, giving you enough breathing room for a pit stop or beverage refill
If a Ritual Monster is not properly Ritual Summoned before being sent to the Graveyard or banished, it cannot be Special Summoned from there by non-Ritual Summoning card effects
Which obviously has everyone scrambling to look up FF characters' birthdays, in anticipation of future spoilers
The Reflection Cycle Alright, you caught me cheating on the first one
"Second Breakfast", or "Eggs Sunny Side Up" ("Eggs" for short), named for the key card Second Sunrise and revolved around using lands and artifacts that could be sacrificed (or in MTG parlance, "cracked") for effects that helped draw cards, recover cards like the Sunrise from the graveyard, generate mana, and all sorts of other bonuses