Assuming this is legal, the card that worries me is the 1-2 mana blue card they could easily reveal that says:
Sorcery: You become the Monarch.
Speculation of course, but I would argue Marchesa is already fringe playable in a 5cc humans list. Against any list that does not play bodies, or any list looking to win all at once like Dredge, an early monarch can be back breaking. I mean I think you could justify fast mana-ing Marchesa out because the longer she is on the board and the longer you control the crown the more powerful the effect becomes.
What is scary is that once someone has the crown it is always a factor in the game. There is no removing it because its an emblem level item. That feels fundamentally very swingy and impactful to the game on a whole.