So... Mega Darkrai is canon and very impressive design after the Z-A DLC. but what about Cresselia? She doesn't even appear in Z-A at all!
I'm very sure, both are the Lunar duo, there must be a balance that can make Cresselia even with Darkrai's strongest form, so if Mega Cresselia doesn't become a thing anytime soon I would be very surprised. I'd say until tomorrow's Pokémon presentation, also known as the final chance for Z-A to recieve new content...

I think Mega Cresselia should still look a bit more different than keeping the same body structure. What I can find geniune is in following Mega Darkrai's giant eye pattern by separating its halo into butterfly-like eyelid wings, as well as actually being able to move her arms, maybe forming a smiling face of sorts?