Mark Gruenwald's basic rationale was outside the Marvel multiverse exists the 'Omniverse' - other universe and multiverses containing what are essentially everything that Marvel either does or doesn't have a licence to, belongs to other company's or mediums, or like The New Universe was something specifically created NOT to be a part of the Marvel Universe.
I don't know if the word or the concept has been used outside of Gruenwald's Quasar series, but the premise is understandable, and it does work as a rationale to explain and perhaps rationalise ALL fictional television, book, and comicbook universes, as being out there somewhere and, as Quasar discovered, are potentially accessible under certain extreme circumstances.
Doctor Who, with the Tardis, is the one character/concept who might well potentially have access to reaching out to a greater Multiverse or 'omniverse'. But really it is pure semantics - an alternate universe is still an alternate universe.
The Dr has now been shown to be Billions of years old, so there must have been travels over history to various worlds and realities never shown nor discussed yet!
was the Dr aware of Superman comic before that Special?