They already announced that they are working on X-Force, why would they label it as a mystery film? Its either they are not sure when to release it or they are keeping it secret or X-Force isn't getting the July 2018 release date. Plus if Wolverine 3 is being released in a non-summer month, shouldn't they do the same for X-Force, given that its not as popular/tested as X-Men/Wolverine.
I do think if the 2018 film is featuring the OT cast. They will have a big returning cast!