The core concept of double-faced cards is transforming from the front face to the back face.

Innistrad introduced DFCs with mainly creatures changing forms from day mode to night mode.

Magic Origins told the stories of Planeswalkers gaining their sparks, starting as creatures and becoming PWs.

In Ixalan, the front side is the journey and the back side is the destination.

Double-faced cards come with two logistical limitations that custom designers may or may not ignore when designing their cards:

• Constantly flipping back and forth is tiring and tedious. Transform cards should only transform once in a while, or only once at all.

• Not as much of an issue now, but DFCs should be used in batches due to printing issues and due to the fact that you've gotta print a checklist card for each set that has them.

They also come with a rules limitation in that the back side cannot use an existing name of a card currently in print. This reduces the shenanigans where you get multiple creatures turning into the same end result.

Thematically, I feel like a lot of people have used it in custom sets to represent Past -> Future, which I think is a fairly logical step. Aside from the occasional flavorful design here and there, I don't see much else thematic space that you can shove DFCs into that isn't already covered by Innistrad, Magic Origins, and Ixalan.

Ha! "Don't see much other space". You're funny :)

Check out Mirroria, a custom cardset consisting of entirely DFCs. Not very sensible in the real world, but in custom cardset world, wow, it's such an amazing exploration of a vast number of different ways to use DFCs. Many of them are "past to future", but that's an overly general category - it includes the Origins planeswalkers and the Ixalan explorations.

I have some opinions about the logistical issues of real-life DFCs (I make it more like 15 logistical issues all told). But I can't deny they're one of the most insanely large veins of design space discovered in the past 10 years.

DFCs can represent any kind of change. I'm long-term working on a new set that uses DFCs to show the transformation between Lorwyn/Shadowmoor - including double-faced supplemental card types e. g. vanguards, schemes, planar cards.

A similar thing could be done for Planar Chaos's "alternate present" theme. What if instead of colorshifting you present alternative versions by having tha alternate on the reverse?

There are many themes of transformation that have used other mechanics in the past - many of them using counters e. g. renown.

In fact thematically the exploration DFCs of Ixalan are reminiscent of Zendikar's quests (not by coincidence since Ixalan is a stand-in for Zendikar's adventure world while the later plane rebuilds) and in a similar fashion you can build upon other existing mechanics.

I used DFCs to repesent t magical girl transformations (though mechanically I made this happen every turn) as well as gods posing as mortals inn before revealing their true form in Skavjando.

I'm very much not a fan of DFC; but as a design space - sure, it's a whole new state - literally doubling the number of things a card can do.

I think an interesting space is actually the one morph played around with. "What's on the back? Nooooot gonna tell you just yet." But done with DFC you can get around the whole "Wait, so this face down is a 22 creature and that one is a land and..." by having the un-morphed state be a whole card on its own.

The limitations and concerns about DFC have mostly to do with paper physical limitations. On digital Magic, its fantastic (for custom designs).

I (and probably others) have thought of being able to cast/play either side (from the hand), long before WotC.

I had also disliked DFCs due to the physical cards, including drafting, the card can be accidentally or deliberately revealed, and if you use the card in your deck then you will need either sleeves or substitute cards. (If there are enough substitute cards included in the packs, this might help a bit.) The idea of DFC is good though, even though there is these problem by the physical cards.

I also think that the rule preventing DFCs face-down is klugy and unnecessary. You can easily represent that a DFC permanent is face-down by either placing the substitute card face-down on top of it, or by turning the opaque sleeve face-down. Similar would be the case for meld cards.

There are many things that could be done with DFC (including transforming, modal, and day/night), though.