Is this too "un-set" feeling? It makes a lot of text boxes silly or pointless, like Goblin Fire Fiend, but they still make sense.

Well, on reviewing the Gatherer further for cards containing "must," this doesn't actually do that much except hose Deadly Allure and flagbearers. I wish Gideon Jura and Alluring Siren used "must," suddenly.

Hm. The problem seems to be that must isn't usually used with mandatory effects (they just say "do this") and also, "may" generally justn turns them off, it's only ones that you want on for you and off for the opponent or similar.

But how about the reverse, a card that says "when you may do something, you must if able"? You'd have to be careful to avoid infinite loops, but I hope it would make at least some cards interesting...

I feel like I would have to meticulously comb the Gatherer to make sure that didn't really break something.

­The Law :)

Ha. The first page already presents a small problem in Abolish, which, if it said "must" instead of "may," would just be weird.

Oh no! I'm unoriginal! :(

I was posting the link to The Law more in response to Jack's proposal of "may is must". Abolish works fine under The Law... it has a mana cost, you just don't have the option of paying it.