I've been thinking about this most of the day, and it's seeming like Mars will be at the very least a part of the content with RoI. As far as fluidity goes, it might make sense for D2 to open with Cabal. Guardians exploring Mars a bit in RoI and then completely in D2 (Cabal), we could see a fluid movement from Mars to Venus with D2 expansion 1(Vex).