I'm pretty sure they will be involved in the next Single player DLC.
Omega is the closest place to the Omega 4 Relay, Cerberus probably took it over in order to have a close access to the relay. They probably made most of their experiment related to the collector base there.
Taking Omega back from Cerberus is beeing nice to Aria, but not something Shepard would do just for the sake of it.
What I'm guessing is that Collectors were Prothean re-engineered to be more easily controlled by Reapers than a regular endoctrinated prothean. If Cerberus is able to control Husks and so on with the Reaper tech, they probably tried/succeeded to do the same with what was left of the collectors.
Something went wrong, Reapers took over the control over the collectors, etc...