To me Other has three roles. First it can act as parts of all elements combined together. Second it can act as unifying traits of all elements. Third it can act as characteristics not exhibited in any element.
The first would be a card that responds to different elemental stimuli in different ways
The second would be a card that could fit in every element
The third is a card that could not fit in any element
Due to the other category and the comprehensive list of elements we have, I do not think we need another element. Maybe other should get more cards but I think we have enough of the 2nd and 3rd types.