Hero Skull - Completely swaps Master Chief(Or Arbiter, for Halo 2 and 3) with your Multiplayer Spartan, in BOTH Cutscenes, and Gameplay.
Heretic - Imitates the effect of the “Hero” Skull, with the exception that Master Chief(or Arbiter) remains in the cutscenes.
And since Halo Infinite is a new game…perhaps the Hero skull could be hidden with some kind of Multiplayer-related easter egg?