It doesnt always have to be an obvious one either
Mardu gives us an embarrassment of great options, all of which play very differently but which work well with the package
From the trade menu, the player has three options, "chest", "check card" and "exchange card"
For its relatively high mana cost of 7GWB, it comes with Ward 3 and offers the player two abilities once it comes into play or attacks: modifying a creatures base power and defense to 7/7, and/or reducing a creature to 1/1
I think this is one of only two cards intially was misprinted, but as the rules changed the misprinted version turned out to be correct (the other would be Daring Apprentice from Mirage, that when printed accidentally missed the now unnecessary "Play this ability as an interrupt.")
What sets Unfinity apart is that a lot of its cards are legal to play in Commander and other eternal formats