But, unlike X-Men, you are only having 4 common characters (not the X-Men plus Magneto, etc). Add like 5 primary Inhumans (only focus on Black Bolt, Medusa, Crystal, Gorgon, and Karnak) plus Lockjaw, and then Maximus.
Shave money from humorous things (like Reed reaching for some toilet paper), hire a cheaper Inhumans cast, and this could work for a reasonable price. Like I said, if this gets X2 numbers, then I can see FOX doing enough to make this. It depends on the gross. FOX coughed it up for X-Men: The Last Stand, and FF made more money than X-Men, so if FF2 equals or outdoes X2, I think they may do it.
Really, the Inhumans are the only story that can top this one visually, and bring in an epic scope.