Namor
In a way I'm a little disappointed that they had to reboot this and we didn't get a third movie out of the franchise. Mainly becaues of that scene near the end where Doom is thrown into the water. I always thought that meant they were going to bring in the Sub-Mariner for the next film, which I thought would be pretty cool.
Although it might be a little too much like the second movie, in that they would be setting up another possible spinoff character. But if you look at the comics, back in the 60s the Fantastic Four launched several spinoffs, so it kind of keeps in tradition. For example, Namor, Warlock, the Surfer, and the Inhumans off the top of my head.
Maybe they should have gone with the Sub-Mariner for the second movie, actually, since they didn't have the guts to depict the Galactus story the way it should have been. The love triangle thing might have been interesting.