Leyline checks a lot of boxes: free, powerful, and wins the game if it hangs around
You can return the Lion to your hand over and over again to basically create the effect of "two mana to give all of your creatures +1/+1." This "build your own Cyclonic Rift" can sweep your opponent's board out of nowhere
Cecil Harvey (Tymna the Weaver) A personal favorite (seriously, I adore Final Fantasy IV), Tymna, the Weaver's design is another card that fits the flavor of the character well
Positioned, as it is, as a casual-friendly experience intended to whittle away downtime, the only difference between spenders and free players will be the speed of completing a collection
Artifact Counters (CTRL to add secondary) Creature Interceptor Mechan Flying When this creature enters, return target artifact or creature card from your graveyard to your hand