As stated before, cards with multiple elemental costs would force a complete rework of the code to implement. Zanz won't do that. Anubis, Pest, Pulverizer and the others don't have multiple elemental costs, but rather a casting cost of one element and a skill cost of another element. Casting costs or skill costs that cost more than one element to play would be a pain to code, and will most likely never happen. However, Zanz has surprised us before, so I typically don't like to make guarantees that something will or will not ever happen. My bet is still no on this one, though.
A 13th mark of Other? Really? That completely takes away the entire flavor of Elements. How would you justify that? Here is the breakdown of why this idea doesn't fit with Zanz's game...
We, the players, are Elementals. As such, we are marked as being affiliated with a specific element. If we wish to change marks, we must go through a process of Metamorphosis (a natural process) to become a different type of elemental. Other cards are unnatural, manufactured items or structures used to tap into raw, unrefined elemental energy. A mark of "other" as you propose would mean that we, as natural Elementals, would go through metamorphosis to come out unaffiliated with an element, and instead would be affiliated with unnatural, manufactured things. That doesn't make sense, does it?
I don't mean to sound rude but the idea just doesn't fit with Elements.