Card not found from Mirrodin Besieged (2011) is a strange one, but perhaps there is an element of a flavor win, because its such a strong subversion of the trope and what does Phyrexia do
Leader cards can use an effect while outside of the main battle, before later being deployed - flipping the card over to reveal new abilities - by paying their cost to join an arena as a unit card
That might also make for a sweet reanimator commander with some clever graveyard manipulation
You can even get some creature power out of card draw with effects like Fists of Flame, Knowledge Is Power, and Chasm Skulker boosting their power or that of your creatures
These are relatively straightforward in terms of setup, and in the right deck wont require any additional deckbuilding beyond swapping out one card
Players won't be excited to throw dice to determine if a 2/2 has Islandwalk or Plainswalk, or if a creature enters to deal zero or two damage