"Omniverse" is a corporate concept. I think it was possibly Mark Gruenwald who voiced the concept for his story where Quasar enters The New Universe in Quasar #31. The New Universe had been clearly stated as having no connection with the Marvel universe whatsoever, so to make the story possible and preserve the original premise to the New Universe Gruenwald devises a situation where while on a mission in the Multiverse on behalf of the Watcher Quasar is blasted while opening a Quantum gateway and is transported out of the Multiverse, to what the Watcher can only speculates is an Omniverse - another sequence of Universes unconnected to his own.
So for the purposes of Doctor Who the concept would and could be used to explain why it is there is no Star Wars, Star Trek, DC, or Marvel universe in his Multiverse - these licenced properties would exist outside his Multiverse... also, see JLAvengers as a possible example.
Great story Quasar #31 by the way, I love Gruenwald's explanations and the fact that the story has not one fight in it. It's all character and concept driven. This story is the spiritual successor to 'Flash of Two Worlds' in my view.
Think that is where they saw the original marvel/DC crossovers happening, outside of both and in the Omniverse!