There's no reason to think this mechanic wouldn't be legal in any eternal format. There's a couple other cards that have been spoiled so far but none that look particularly interesting.
My first thought is to compare Monarch to Edric, Spymaster of Trest. Being the monarch is pretty much the exact opposite - you're rewarded for not being hit instead of hitting. Being the monarch certainly has more risk involved. You're probably playing a fairly defensive deck, so if they hit you and steal the crown, it may be difficult to take back.
I'm thinking starting somewhere like 4 Dark Confidant, 4 Swords to Plowshares, 4 Lightning Bolt, and some number of either Moat or Glacial Chasm. edit: Maybe Ghostly Prison?