But it can't be the future, since it is actually the past realative to the people on the Island in the main storyline.
Remember, the events currently going on in the main storyline take place 3+ years after the crash of Flight 815 (so sometime in 2007-08).
The events happening in the "flash sideways" are occurring right after flight 815 lands in LA (so in 2004).
So at best, the FS's are an alternate history where the plane never crashed, but they can't be "flash forwards" since they take place in the past...
Unless all of the people currently in the main storyline all end up going back in time once again, lose their memories of all the events happening after the plane crash, and then get on Flight 815 and have it not crash.
Then the "flash sideways" could be "flash forwards" of the characters physically, but not chronologically. Or rather, they are "flash forwards" of them personally, but not of the universal timeline itself (since it would actually be "flashbacks" according to the universal timeline).