My theory is that the sunbreakers are going to team up with the cabal and let the cabal use the power of their forge on mercury to get better technology. The drill in rubicon wastes is most likely drilling to charlmanges vault, a rumored area for a raid in pre scrapped destiny story that involved rogue guardians (see the connections). Zavala will probably be the main quest giver due to his beef with the sunbreakers. I have a huge theory about this on a different post I made a while back.