I wouldn't want Blackest Night in an omnibus that only included the Green Lantern Issues. You need at least the main event as well and preferably the GLC issues. With the solicit for the Vol 1 omni only including the GL issues of Sinestro Corp War I'm pretty concerned the GL omni's are going to be subpar and miss vital chunks of the overall story. The GL story grew too big for just the Geoff Johns issues and leaving out stuff not written by him is short-changing the overall epic.
For Blackest Night it would be good if they release a Blackest Night omni on it's own with the core series, the GL and GLC issues and the various tie ins. There is enough there for an omni to cover all that.
Post Blackest Night they really need to include GLC and Emerald Warriors as those stories paralleled the GL arc and provided some important background. Further on New Guardians also was closely tied in. Red Lanterns isn't necessary though.
Should probably add imo to all of the above.