Woah, this is awesome and I'm certainly using this the next time I do a homebrew camapign together with my players.
An idea has crepped into my mind: You increase the AP cost of actions that (negatively) affect another players creations, I guess to de-incentivise potential grieve. But I feel like in "drama" lies the essence of good stories.
So as an alternativ idea, instead of increasing the cost, take inspiration from hostile invokes/compels in Fate: If your action (negatively) impacts another player one of the spend AP is given to the affected player/steward instead of being spend and removed from the game.