Heres what I propose for the shia and sunni symbols

Sunni: just a crecent, this was originally a pagan arab symbol.

Shi'a: either the Lion of Allah or Zulfikar (the sword of Mohammed which was given to Ali and then was present at Karbala with Husayn)

http://www.amaana.org/alilion.gif

also, I belive that there should be a third branch included, the Nizari's or as today they are called the Ismaili's. They belived that the last Imam was the
7th Imam, and that Nizar was the correct succesor of Mohammed. Ill find out their symbol later. They in fact were the "assassins" and the word we know today originated from their name. Although they were banished to Iran, they went and served as killers for many emirs and caliphs. They were thought to be "hashishin" because some belived that they would take hashish to put them in the killing mood.