1. Man-Thing
With recent name drops to the character himself and some of his supporting cast in Iron Man 3 and Agents of S.H.I.E.L.D, one can only assume that Marvel does in fact have the movie rights back to Man-Thing even though last we had heard he was still with Lionsgate. The only way to know for sure is if someone from Disney or Kevin Feige himself officially confirms that Man-Thing is back with Marvel or not. Assuming however that Man-Thing is still with Lionsgate, I think Man-Thing should return to Marvel because of the fact that Lionsgate clearly has no plans for him in near future anyways on top of that the 2005 film of Man-Thing turned out less than stellar. Man-Thing would be better utilized in the current MCU as a villain to Black Widow (assuming she does get her own film) or even a supporting character to someone like Dr. Strange when and if they decide to pull the trigger on producing that movie.
2. Namor
Currently Namor's movie rights are with Universal and we have seen exactly 0 films starring him. I personally am not a huge fan of his but there are many out there that are. Ever since the 2003 Hulk movie Universal has been dead silent when it comes to using Marvel characters. If Universal does not use him soon I can only imagine Marvel will get the rights back to him some time in 2016 and that's being generous to Universal at this point.
3
. Fantastic Four
Out of my 3 choices Fantastic Four is the team that I want back with Marvel the most. The first film back in 2005 was bad but did make $330 Million at the box office against a budget of $100 Million (that's not including the advertisement budget as well as the fact that theaters get a cut of the admission price) at the end of the day however it did make money so a sequel was ordered and somehow someway they managed to make the sequel even worse! Granted I liked the cast overall for F4 but the script was beyond terrible. Instead of just hiring new writers Fox studios decided to reboot the franchise all together using some of the worst casting decisions I have ever seen! At the end of the day regardless of whether or not F4 turns out to be a good movie I just don't like them. To me they are a boring team and just doesn't translate well to film. So why do I want them back with Marvel? Because of 4 specific characters: Dr. Doom, Silver Surfer, Galactus and Annihilus! All of these characters came with F4 in a package deal made between Fox and Marvel when Marvel sold the rights to F4 away. Can you imagine just how sweet the MCU could be after Guardians of the Galaxy premiers if they had characters like Silver Surfer and Galactus that could show up down the road in Phase 4 or 5? Dr. Doom could be a great threat to The Avengers and F4 themselves would benefit greatly just from being a part of the MCU.
What about X-Men and Spider-Man?
I know some of you may be mad that I did not include Spider-Man and X-Men on the list of characters that I want to see back with Marvel but hear me out for a second. Spider-Man overall has not been in bad hands with Sony I was not pleased about not getting a Spider-Man 4 especially since I actually liked Spider-Man 3 (Yes I did just type that) and I also do not like the way the new Spider-Man looks in the reboot however Spider-Man is a great solo character with so many villains and great stories that have yet to be seen on screen and as much as I like the MCU we would not be able to see those great stories be told with Marvel trying so hard to maintain a shared universe with so many different characters already. X-Men on the other hand I have a love-hate feel for on one hand I really did like X2 and enjoyed First Class however every other movie either feel flat or was just plain bad. Both Wolverine films were horrible and X-Men 3 was beyond bad. However despite all that I still think Fox can make X-Men work besides I have a hard time seeing X-Men who are hated for being mutants being in a world where a "monster" like Hulk is beloved by the general public. X-Men has so many characters and so many villains that I believe Fox will eventually find its footing and bring about good films for years down the road.