5) Namor The Sub-Mariner
One of the bad things about Marvel properties being spread out over different studios (e.g. Fox have the X-Men) is that it is sometimes unclear who owns which characters. As such, legal issues stop any of the studios involved from bringing them to the screen. One example of this is Namor The Sub-Mariner, whose rights rest somewhere between Marvel and Universal.
This is a shame, as Namor is a huge presence in the Marvel comics universe. In fact, he’s the company’s first ever hero, introduced in 1939 back when they were still Timely Comics. Like the more famous Aquaman, he is a merman who is king of Atlantis. Unlike Aquaman though, he’s often presented as an anti-hero or an outright antagonist. Seeing as Marvel made a deal with Sony for Spider-Man, maybe they can sort out Namor’s rights in a similar way in the near future.