I think Marsden has a slight advantage over the actors vying for the role in that he has worked with Marvel before.
Chris Evan, Ryan Reynolds, Will Yun Lee, Ray Stevenson, Sam Elliot, Jon Favreau, Ben Foster, Rebecca Romijin and Idris Elba are all actors that have starred in Two unrelated Marvel properties.
Then we have people like Zachary Levi (attached to play Fandral in Thor, Now brought in to play him in TDW) and Adewale (in talks with Marvel back in '09 to play Black Panther, now set to play Kurse) who Marvel developed a relationship with, and when the appropriate opportunity arose, they went back to both these actors.
Then we have people like Thoms Jane, Natalie Portman and Edward Norton who previously turned down roles in a Marvel film, but later return to work with Marvel.
Then we have Emily Blunt who had left Marvel twice! First with Black Widow in Iron Man 2 then Peggy Carter in CATFA, and is still rumored as a possibility for another character (possibly Ms Marvel in another Marvel film).
Point is, If Marvel developed a good solid rapport with a person, its likely that might want to work with that person again... That's why I think Marsden might have a better shot then most of you presume.
Although I have to admit, I want to see him return as Cyclops, in DOFP and possibly future X-Men installments especially if they decide to do a X4 in addition to a X:FC2