I really like those covers for Immortal Hulk #2 & 3. And our multiversal antagonist/threat is... The One Below All. Hmm... knowing Al Ewing, I would not be surprised if it's some kind of dark counterpart to or darker aspect of the One Above All/All-God.
Maybe the One Below All is the All-Devil/All-Demon. I like that Al Ewing hasn't revealed the One Below All yet, just the name. Would be cool if he's some powerful, terrifying entity like Oblivion or Death. Maybe something along the lines of the Logos or Aspirants or the Proemial Gods or the entities of JL: No Justice.