Now im sure someone has sorta though of an idea like this. But i think i got a sweet new idea.
Instead of just playing an insurrectionist it would be even better if you played as a UNSC Marine, and an insurrectionist. Say on one of the outer colonial planets. You can play the role of a Marine tasked to take out insurrectionists and play an insurrectionist who wants nothing more than to be separated from the UNSC and gain freedom for his people. That way you get to be put into this plot where it seems that both people are in the wrong, but also have their own reasons for being right/doing what they are doing. Maybe the Marine you play will have some bad history with insurrectionists who attacked where he lived and killed some of those he loved. And on the insurrectionists side it can show a darker side of how the UNSC can be so un-diplomat allowing players to really pick what side they feel is right. BUT. Out of no where during a firefight between the insurrectionists and Marines they get ambushed by the Covenant and are forced to join forces and fight this new enemy. They could get SGT. Johnson (which would be AWESOME) if they can even get the voice actor back. And have some Spartan IIs make an appearance. HECK, John 117 can show up. That would be sweet.