Here's an idea I had (or basically stole from other games) when I was thinking about all the possible slot mechanics:
Location Slot
Both players have a location slot, but when one person plays a location card, both slots get filled (replacing the card that was there previously). Idea is of course that elementals can dictate the place where the fight takes place, giving them a possible advantage over their opponent. For example you could play "Burning Island" that slowly damages all the creatures (you don't care because yours have high HP), or maybe "Tower of Anti-Magic" that makes spells cost +2 quanta (you don't care because you have no spells). Deckbuilding-wise, the cool part would be to make the choice between taking lots of location cards and being able to dictate where the fight takes place, or taking no location cards and praying that the location of your opponents choosing doesn't destroy you. What would make these location cards different from regular permanents is that they would be more powerful since they are easier to counter, and they would always effect both players.
Why I'm telling this? Well, Field slot suggestion is surprisingly close to what I had in mind. The card mechanics are exactly the style I was thinking so I was really excited when I started to read about it. Unfortunately the two cards active thing was not that great imo. I kind of dislike the fact that it's complicated like that and requires that extra click. One card is simpler and better imo. I'll still vote it though.
I also liked Familiar and Rune. Latter maybe has a bit more interesting mechanic.
Som yea.. I picked those 3. I think the overall quality of submissions was surprisingly good and I could have easily picked more.