This would be cool, but I think he is tied to the no time to explain. That ghost we got has been seen as classified in the archive and though to be related to no time to explain. And him being a big part would be really cool but I don't think this is the case, since you scan his bones at the end of that quest normally. I believe that those fragments showed us that the Vex knew they couldn't defeat Oryx (they have been at it for a long time) and so they basically got us to do it for them. He says "they won't thank us, they won't spare the city" so I think that the Vex dlc will have to do with them going on the offensive against us now that Oryx is no longer a threat.