The problem with him being in Avengers 2 would be that his invasion of New York with Atlantians on flying ships, with giant sea monsters too, would seem too similar to Loki and his "skrulls". So the FF might be better because it would be different. But the FF is still with the studio that made the bad films so we would have to wait till the rights go back. Wouldn't want them to mess with Namor like they messed with Doom or Galactus.
But I would really like to see Namor 1st introduced to everyone as a major bad guy. Then he is given his own film to show he is a good guy. To me personally, if he was 1st introduced in his own film he would probably be shown as a good guy and then showing any dark side to him might seem less dark because everyone knows him as a good guy. If everyone knows him as a bad guy 1st it would seem that the darkness is part of his true character and not just him in a bad state of mind.
And he should be in a movie that would show his power. If he is in a movie against Spider-Man or even Iron Man I think it wouldn't show how much of a threat he is, just make him seem like a smaller threat like Iron Monger or Green Goblin. But if he was in a movie against the Avengers, FF, or SHIELD, that would be a lot of opposition to him.
I prefer tragic villains.
The thought of a character who wanted to co-exist with his human counterparts, as alien as they felt to him, turning over a tragic event is far more dramatic than just introducing him as an already evil entity.
He wants to be human, he wants to leave the waterworld behind, but the people up above do him wrong, maybe harm his family, maybe murder his human love interest, etc...something tragic enough to turn his heart to stone and make him develop a pronounced hate for mankind.
I don't want him facing the Fantastic Four. He's too big for 'em. This is a guy that's invading Earth from
within & that can surround dryland a hundred times faster & with more troops than Loki will in this year's Avengers film.
You need the world's mightiest heroes to take him down. You need Thor. You need Hulk. You need Giant-Man. You need The Avengers.
Then, when all is said and done and enough people & atlanteans have died, you can have Namor & the people of Earth come to a truce. He realizes its a war neither party can win & returns to the sea as neither ally nor foe.
The character that comes to mind when I think of adapting Namor for film is Walter. E. Kurtz from Apocalypse Now. He starts off a good man, trying to make a real difference, but loses faith in mankind & turns on his own during the war. At the end though, he comes to the realization that he himself became part of the problem through his wrath & urges Willard to kill him.
There's so much potential for this character. Hopefully they won't mess him up.