Really, nobody needs a solo film before they move on to the Justice League movie. A Justice League movie right away could work. It just needs to be very well written and not cluster-****ed.
And really, is it really so bad that we're not getting an overload of DC characters right now? Think about it. Marvel is bombarding us with Iron Man, Iron Man 2, The Incredible Hulk, Thor, Captain America, and The Avengers. Not to mention X-Men, Spider-Man, Wolverine, Ghost Rider... all of this in literally 3 years.
I don't think it would be wise for DC to suddenly add to the already exhausting line-up of superhero movies that Marvel is bombarding us with.
Here's the most important point to consider: There will come a time when Marvel will be exactly like DC is right now. At some point, Marvel will run out of options. This is like a giant teeter-totter. Marvel is up, DC is down, and vice versa.
Once The Avengers is finished, it won't be so simple to make a Thor 2, or a Captain America 2, and it won't be so simple to reboot The Incredible Hulk for yet a third time. And it won't be so simple to reboot Iron Man after Robert Downey Jr. is done after Iron Man 3. We may get a sequel to The Avengers, but it won't be so easy splitting them all up again. Seriously imagine how difficult this could be. Robert Downey Jr. is the Iron Man franchise. Once he leaves, the Iron Man franchise will disappear for years, or at the very least, not be as successful.
As far as the X-Men, we got a prequel with Wolverine, and then First Class was a prequel to that prequel. Is it really going to be easy making an X-Men 4 that takes place after X-Men 3: The Last Stand? My guess is that this franchise is in serious trouble too. You can't just make five movies in the same continuity, and then, yet again, make a second attempt at a Wolverine movie in 2012. The whole story arc of this franchise has hit a giant PAUSE button. They're not furthering the story - they're going back, and then back again, and then back yet again. It's been 5 years since The Last Stand, and there's no plan to further that story more. It's a giant mess.
The only thing that Marvel could really have going for it after 2012 is the new Spider-Man... if that's a success. I can't imagine anyone flocking to a Nick Fury movie. (And yes, I'm well aware that X-Men is Fox and Spider-Man is Sony, but it's still relevant.)
So, in conclusion (as they say), Marvel might have a serious problem. They're super-successful right now, I'll admit. They're kicking DC in the ass. But this huge success might suddenly hit a brick wall.
Once this crisis occurs, DC will be there to get its turn. It's just going to take a few years. I really don't view DC as "having a problem" right now, honestly. They just need to get all of this organized, and then it'll be their turn. Like I said - it's a teeter-totter.