A Geeky Viewpoint

DISCLAIMER - These are rumors that I cannot personally confirm however they come from individuals who have proven to be fairly reliable.   This morning a usually VERY reliable Hollywood insider posted that Marvel extended an olive branch to both 20th Century Fox and Sony / Columbia Pictures concerning the two studios Marvel Intellectual Properties controlled by these studios.   Several other sources have posted rumors too.   The problem with the additional rumors is whether or not they are just speculating on what was said int he first rumor or do they have a source following up on what is said.

There have been many posts and rumors about crossovers but this one is slightly different as it cast Marvel Studios (Marvel Entertainment) as the one presenting the Olive Branch. 

According to the rumors Kevin Feige personally meet with his counter-parts at 20th Century Fox and Sony to inform them of Marvel Studio's plans for Phase 3 in their offices prior to the Marvel Fan Event of Tuesday.   The rumor continues that he went beyond saying here is what Marvel Studios is planning but to included some of their decision points and other options.   In simple terms he included the why and how for Phase 3 not just the when and what.

The rumor continues that Kevin Feige personally extended the offer to include characters from all three of the IP's in the Marvel Cinematic Universe with potential for characters from all three IPs to appear in Captain America: Civil War and additional films in Phase 3.
In return a number of characters from Marvel's stable would be made available to both 20th Century Fox and Sony.  This includes the ability (requirement?) to use the same actor as in the MCU (TV).   Firestorm, Nova, Iron Fist, Tigra, Jessica Jones, Daredevil and many others have all appeared with Spiderman over the years in the Animated TV shows.   Hulk and Wolverine have a long history as well.

This is where the different rumors diverge ...
20th Century Fox either said "thanks but no thanks" OR asked if "this could be limited to Fantastic Four" OR took it under advisement.

Sony / Columbia Pictures at the moment seems desperate to save their Spiderman Universe, with rumors a plenty concerning what's going on.   If Avi Arad was removed from the picture I am guessing Sony would have already signed on the dotted line.   Rumors suggesting Marvel would finance the Spiderman Films, use Spiderman in meaningful ways in the MCU and offering up complementary characters and villains would be incredible for Sony / Columbia and solve multiple problems.

SPECULATION - Here is my rub on this.  
Sony / Columbia have seen the writing on the wall, a Spiderman reboot would be a killer finically for the project.   Sony costs would increase and the interest on the financing of this might get kicked up a couple of notches (Columbia is not self financing rather they take out a line of credit loan to bankroll most big budget films).  The $250 million they spent on The Amazing Spiderman II might be closer to $300 million for the next project.   The financing offered by Marvel would essential reduce the cost to Sony by $25 to $50 million.  
The only real question in this is who gets to talk to Avi Arad, who's ego is already battered and bruised, and claims he is being stepped on.   How much control does he have?
Let's be honest adding Spiderman to Captain America: Civil War would be a major victory for Sony and Fans.   It also creates the perfect man in the middle, something Black Panther will have problems doing.   This is a Win Win situation for everyone not named Avi Arad.
  
20th Century Fox has been successful with the X-Men universe but it is very Wolverine centric.   I don't see that changing in the near term with X-Men: Apocalypse already in the works and The Wolverine 2 in the pipeline.   The two standalone films Deadpool and Gambit are a different story.   Those are fairly big risks.
The Fantastic Four looks like it is a ticking time-bomb, but hey even the Teenage Mutant Ninja Turtles reboot got a sequel.  
The Wolverine earned $132 Million Domestically and $415 Million World Wide roughly half the standalone MCU films.   Would adding Mark Ruffalo to the cast of The Wolverine 2 bring in double that amount, maybe even triple?  
Kevin Feige suggested that Hulk had a very important place in MCU, Mark Ruffalo would appear in the Avengers films and have meaningful roles in additional films.  How does Hulk fit into Doctor Strange or Captain Marvel? 

This goes beyond Phase 3, Isaac Perlmutter CEO/President of Marvel Entertainment (Kevin Feige's Boss) has been vocal about wanting to do Secret Wars and Avengers vs. X-Men [AvX] on film.  He has threatened to use the whip on occasion to get what he wanted, is he now suggesting that carrot might work?