Okay, so Harbinger was using the Collectors to make a Human/Reaper hybrid out of colonist paste. I'm still not too clear on why, though; it seemed to me like there were already plenty of Reapers out there. My best guess is that the hybrid was supposed to replace Sovereign and go fire up the Citadel relay, although it looked like the Reapers were well on their way towards the galaxy already...hmm. Anybody got a clue?