If you gain control of another player's creature until end of turn and exert it, and then that player regains control of it, it will untap during that player's untap step
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So no color identity means no mana
Honestly, this maybe should be a demon typal deck, assuming you can slot in enough sacrifice fodder to cheat in your demons
Lifegain is a popular mechanic among Commander players (at least very much among the players that I know), and a simple 4-drop that not only shuts off all lifegain but reverses it into life loss seems like an extremely powerful effect to me
So, aside from being powerful in its own right, Vivi Ornitier has a lot of support that you can use to create a very effective spellslinger deck