A lot of us are already betrayers, sure, siths do good stuff once in a while, but a sith who allows a jedi to escape, helps the poor and ends up at lvl 40-50 with no darkside points whatsoever .... thats more of a betrayer than a sith. The same thing could be said for Jedi serviing the republic with the opposite tag.
It would have to be an expansion pack allowing you to betray, perhaps named: The betrayal ..... it could be an expansion based on exclusive missions to help betrayers or the opposite, saving your empire/republic. In the end everyone would have something to do and it wouldnt be a bad story to start with ... who betrays who?