I was sitting around and thinking, what if Marvel got the film rights back to all of their characters? Who would they use? What films would they make with them? and that's when something interesting came to my mind. If Marvel studios got the rights back to the X-Men, Fantastic Four, and Spider-Man, I speculate that they'd have little interest in making any of those films, since they are focusing on The Avengers. What they would want is particular characters, such as Wolverine, Spider-Man, Silver Surfer, and Galactus for The Avengers, Professor X and Mr. Fantastic for the Illuminati, and Norman Osborn and Venom for the Dark Avengers. It would be a shame to not see a X-Men, Fantastic Four, or Spider-Man film for a while, just so that Marvel can have a handful of characters.
As I continued to sit, I remembered how a few days earlier I watched The Last Stand. As I thought about it, I wondered how much better it could have been if they had more time. With fear of the film rights to X-Men and Fantastic Four going back to Marvel Studios, if they wait too long, films like X3 are forced out. Fox, if put in the situation again will force another X3 out rather than let the rights go back(Daredevil is the exception, but remember they rather let those rights go back than the rights to the Fantastic Four). Sony was put in a similar situation with The Amazing Spider-Man but in my own opinion, they handle it pretty well, but given more time, They could have done much better.
[UPDATE] To be clear from what I wrote above, Fox and Sony will NEVER EVER let the rights to their Marvel characters to go back to Marvel studios, so remember that before continuing reading.
All three studios have a problem which could be fixed with one deal. Marvel should negotiate a new contract with Fox and Sony, allowing them to hold on to the rights of X-men, Fantastic Four, and Spider-Man as long as they like, without fear of the rights going back to them if they don't force a film out in time. Yes, you read it right, but there is a loop hole, that allows Marvel to use any of the charcters they like, at anytime and for any reason. With this deal, we won't have to worry about Fox or Sony forcing out potentially bad films, just to keep the rights to their Marvel characters, and Marvel Studios can finally use almost every character they want in future MCU films.
[UPDATE 2] Let's not forget that with the studios sharing the characters, it could potentially bring more popularity to these characters and comicbook films in general. The more popular these characters become could lead to higher box office numbers from all the studios, which something I thing Marve studios, Fox, and Sony would be interested in, if they could look past their differences.
[UPDATE 3] Sorry if I didn't make it clear, and hopefully this will be my last update, but in this shared deal, it will not effect the already established univeres of Fox and Sony. If Marvel choose to use any of the characters owned by Fox or Sony, it will be their own version, existing outside of the Fox and Sony universe. So Fox and Sony can continue making films as they have planned or will plan without Marvel swooping in screwing their universe all up. In short, they will not be connected.