I think ultimately a Morrison Batman will happen.
It's easy to give my top ten (or twenty or thirty) most want omnibuses. But if you ask me for a likely omnibus, that's where I get confused. What's reasonable to me doesn't seem to have any basis in whats reasonable for DC. I see omnibuses as mostly to a collector's market, I don't think too many people are just randomly buying omnibuses. So in that regard, just about any run seems reasonable to me. I'm actually surprised that DC doesn't have more classic material in print. I can't figure them out.
Take the 52 weekly series omnibus. If memory serves, that was a good seller for them. I would think by that mark, Countdown would get an omnibus. Similar series, maybe not quite as critically received, but I liked it and people would buy it. Not out. Or the Jack Kirby New Gods stuff--why wouldn't you release that in a traditional omnibus?
Hard for me to guess cause I really don't know their criteria.