What happens if you Xenograft for Human, followed by Moonmist?

I suppose the game would notice that an instant is on the battlefield and promptly put it into the graveyard. I've read about that, but didn't know of a case where it could actually happen.

I'm making a new rule: instants and sorceries on the battlefield are put into the graveyard as a SBE. Right now, the game doesn't actually do anything about it.

I don't think you want your rule 3 to be worded the way it is. At the moment it says:

> Whenever a DFC (double-faced card) changes zones, it enters the new zone with its day face visible.

But that will hose this cycle. These will transform as they begin resolving, i.e. as they're still on the stack; then resolve as creature spells; part of which is putting them onto the battlefield. Except rule 3 says they'll ETB on the day side, and then rule 1 or 2 says they won't actually ETB at all but will just go to the graveyard.

I think in fact that something similar to your rule 3 is already implicit in the rules (110.6b or similar), and you'd need instead to add exceptions to 711.2a, 711.5 and 400.7.

I noticed that. I was planning on adding an exception when it goes from the stack to the battlefield. I'll look into the others you mentioned as well.

I guess you could get around the "instants and sorceries on the battlefield" issue by making them all Auras with ETB abilities, but then they wouldn't be as clean.

If you did go with Auras, you could also make the ability "If ~ would be countered, instead it ETBs transformed", which would get rid of the stack issue.

A couple more suggestions for the rules:

> If an instant or sorcery card would enter the battlefield, it doesn't. Rather, it's put into its owner's graveyard as a SBE (state-based effect).

This one should probably not be an SBE. It should probably say something like:

> If an instant or sorcery card would enter the battlefield from any zone other than the stack, instead it remains where it is. If an instant or sorcery card would enter the battlefield from the stack, instead it's put into its owner's graveyard.

Also, your rule 6 is quite dubious:

> When a DFC becomes a copy of a SFC (single-faced card), only the visible face becomes a copy. The other face is unaffected. The DFC may still transform.

This is rather at odds with the way most continuous effects apply. In particular, it's different to the way that morph and flip cards work, and the way that DFCs currently work. If a card gets flipped or unmorphed while it's a copy of something else, the "underlying" characteristics change, but the copy effect still applies for the duration it would apply. So if you cast Break Open on my face-down Venomspout Brackus, and in response I Cytoshape it into a 3/3, then it'll become a 3/3 effect "over the top of" its base 2/2 stats; then when Break Open resolves, it'll still be 3/3 "over the top of" its base 5/5 stats. I'd expect DFCs to work the same way, and under current rules they do. Why do you want to change this?

I hadn't realized that it was covered under the existing rules. I don't want to change things just to change things.