A good one to do is The Brotherhood Of Evil Mutants, since we have a good portion of them. The regular symbol is Magneto's helmet in a circle, but that could get confused with his die. The Sidekick dice could be purple with red to avoid that.
There is an alternative symbol, but I don't know if its official: a 6 fingered hand in a star.
Both Brotherhood and X-men could simply use generic Mutants.