It seems there is sometimes the need for "reversed" ternary plots, i.e. that is simplest explained at http://tex.stackexchange.com/questions/213278/reverse-axes-of-pgfplots-ternary-plot. Here is another question that wants this feature http://tex.stackexchange.com/questions/192043/flip-x-and-y-axis-of-ternary-graph.
I think this could be implemented quite easy with a key named ternary dir=normal|reverse. But when you are at it anyway, maybe you could also add another option like ternary x pos=bottom|left|right, if you know what I mean. Then I think all possible needs should be satisfied (except one ones the peak at the bottom instead of the top).
