If we disregard code difficulties: (since new elements are basically slightly less difficult to code than Hybrids in Cygnia)
The key problem with adding new Elements in terms of design is that certain cards like Immolation and Nova already hinge on the large amount of quanta advantage they generate for "1 of each element". This makes it increasingly difficult to cost Other cards fairly - if you look at previous Other-cost designs in this section, you'll notice that they're either inefficient, too powerful, or struggle with trying to find a fair balance in between the extremes. (If you compare a game like MtG, which has 5 colors, you'll notice that their colorless cards tend to have an efficient ratio closer to their colored cards, partially because they can't 'burst' as much resources as a game like EtG could.)*
Since
and
already represent something in relation to other elements (typically Other-cost cards), I'd strongly suggest going with
or a symbol of your own.
P.S. - Another significant problem would be defining distinct mechanics for a 13th Element, though given that 12 Elements already implies mechanics will likely be shared in the long (assuming an infinitely expanding cardpool), this is less of a problem here than the Nova/Immolation issue.
