4 Hitman vol 1 ( The Demon*Annual #2,*Batman Chronicles*#4, and*Hitman*#1–33 and Annual #1)
Another one of Dc's "mature" titles. This one by Garth Ennis. Hitman was introduced in Ennis' less popular comic The Demon and quickly got his own ongoing. This title about a superpowered Hitman ran for 60 issues and is considered to be one of Ennis' best runs. This first volume collects Hitman's first 2 appearances in The Demon and Batman Chronicles, the first 33 issues of the ongoing and the first and only annual.
3 JLA by Grant Morrison( JLA #1-41, New Year's Evil: Prometheus,*JLA/Wildcats,*JLA Secret Files*#1, JLA: Classified #1–3, JLA: Earth 2 and #1.000.000)
The top 3 opens with probably one of the most likely titles to get an omnibus soon. In the late 90s Grant Morrison took over the Justice League and it became one of the most popular titles of that time. It is notable that while JLI gained popularity because it used C-listers, the first few issues of JLA feature only the definite A-listers. Later, other characters would join the mix though. As we all know, Morrison's runs are very likely to be omni'd, add that to the fact that this is probably one of his most mainstream and best selling comic and it makes you wonder why Dc hasn't done this yet. This omnibus would in the league of Invisibles and Doom Patrol cause it collects not only all 41 JLA issues by Morrison but also the #1.000.000 issue, the 3 issue mini JLA Classified, the OGN Earth 2, a special, a crossover and a Secret Files issue making it 50 issues big.