8. The Multiverse
While TV shows like Agents of S.H.I.E.L.D. and Daredevil take place in the Marvel Cinematic Universe, scheduling issues between those make any sort of crossovers highly unlikely. However, because Warner Bros. aren't as concerned with everything they produce being set in the same world, a "Multiverse" has been created which allows multiple versions of the same characters to exist in different forms at the same time. As a result, there's really nothing to say that we won't one day get some sort of big screen adaptation of Crisis on Infinite Earths, a movie which could unite these heroes. That's actually far more exciting than the prospect of The Defenders MAYBE showing up in Avengers: Infinity War as it would be such a huge event, and with The Flash about to invade Supergirl's world, it seems the studio is already testing the waters.