How would DC returnning to the multiverse concept be a threat to the current DCU? I doubt that DC is going to "split" the current Earth-DC into Earths 1, 2, 4, S, & X. It's more likely that they would establish that the current DCU is either another parallel universe of the "Pre-Crisis" DC Multiverse, or that the current DCU is part of an entirely separate multiverse. There is absolutely no guarentee that a new DC Multiverse means the return of any of the DC Multiverses seen during or before the Crisis on Infinite Earths.
I'm actually curious what the plans by Johns, Morrison, Busiek, Simone and the rest are with regards to the multiverse, if indeed that is their plan. My intuition is that it isn't going to go back to the Pre-Crisis status quo with heroes on different earths, but that it will be something stranger and less predictable.
Best Case Scenario is a multiverse is restored, but it is a more intriguing and different kind of animal that makes different stories possible. Note their specific wording, not that the multiverse will come back, but "the multiverse still exists." What does that MEAN?
As I've never thought much of Captain Marvel, I hardly agree he needs a whole earth all to himself because he has to be the SPESHULEST. JLA/AVENGERS established that DC Earth is bigger in area than our earth, and there's room enough for both characters.
I don't blame the singleverse concept for Captain Marvel sinking instead of swimming in recent times, because Cap was floundering without direction even in the days of the multiverse (though I will admit I did enjoy the issue of DC COMICS PRESENTS with Superman and the Marvels vs. Kull).
And anyway, Black Adam seems to be doing fine at present thanks to Johns's ingenious use of him. What's Cap's problem, anyway?
Sure, I'm not the target audience for the Marvel Family and perhaps my criticism can be discounted for that reason (if you ask me the two highest points in comics history, it would be the Steve Gerber DEFENDERS and the Englehart DETECTIVE COMICS), and coming down on terminally retarded, condescending idiocy like the Goatman feels like kicking a dog because it can't do calculus.
Still, the reason I find the Marvels frustrating is not because they are choked by bad ideas that talk down to the audience...but because there are so many GOOD ideas in there that can't find expression...and that perhaps by placing them on the same grounded, science fictiony earth as the JLA and the Green Lanterns and so on, they can be developed to their full potential and played "straight."
Shazamo for example. He has a great look and terrible powers, and Thanos among others has certainly proven that an evil god can make a formidable supervillain. Shazamo has the potential to be truly terrifying an antagonist.
THIS is why I'm not 100% certain giving Cap and his buddies his own earth is good idea: place him back in his old, cartoony context and you can't utilize it all to its full potential.