X-Men Forever begs a question (for me at least): Is it still considered fan-fiction if it's written by the author who can nominally be considered the creators of the characters? Some of the qualities of (mediocre at best) fanfiction present: established relationships rewritten to authorial preferences, the laws of science and genre convention convienently ignored for nonsensical dramatic moments, and a general sense that the whole point of the exercise is to demonstrate your own superiority to the works of others.
Now, there are those, particularly Alan David Doane that would argue that all (or nearly all) of modern Big Two comics output would be considered fanfiction, and has even coined the term "The Fanfiction Age". I'm not entirely sure he's wrong; there's certainly enough evidence out there to make a good case for it.
However, there is an important difference here: that Chris Claremont is the definitive X-Men writer. All the characters, even those developed before his tenure, were ciphers until his long and storied run. He developed the personalities, the dynamics, and the stories that Marvel has continued to mine for nearly 20 years since he left the property, including for a lucrative film franchising of the property. And Claremont's reward has been, at best, bittersweet: he's gotten his share of work since then, and has had various "golden parachute" titles that Marvel has put out simply on the strength of his name. On the other hand, I can easily imagine him shortly after his heart troubles, sitting at a TV watching "Wolverine and the X-Men" (or rather as I like to think of it - "X-Men: Days of Future Past - the Animated Series") and wondering to himself if he wouldn't have been far better off bringing his ideas to an idea that he could have fully owned and profited from. But that's life in Big Two comics.
All that said, X-Men Forever is not necessarily all that bad; it's certainly in line with the type of X-men comics were coming out in the early 90s, and in its own way, the freedom Claremont has to shake up the status quo whatever way he damn well pleases gives an interesting insight or three into how he views the characters. And it's certainly as valid as the current bad fanfiction being put out by NuMarvel.