I doubt it will be Ares.
Honestly, this is the era of the comic book movie. If there is going to a "next in line" for the Marvel posterboy title, it is going to be someone with a blockbuster movie franchise. That said, Spiderman/Wolverine are still big even in the movie world. I doubt they will be overtaken in popularity so much as other characters will simply catch up to them.
Ok i have seen these ideas come and go in the early 80's it was surfing with the alien and superman was boss. By the late 80's it was all about spiderman and wolverine and has remained so since. The late 80's and early 90's saw Gambit become a huge fan favorite where deadpool seems to have stolen a little glory. Cyclops seems to finally be surging with the aid of descent writing. Personally I am telling you if Singer gets Channing Tatum in the role, ok women have always flocked to the ida of Gambit, nw you add thier fantasy date as the lead role and guess what date night becomes one with an x man.