I thought that my time fic was going to be jossed by The Last Man, but it turned out that mine is so far different that I can still write it (and Jayne Perry has kindly agreed to co-write it with me, though I suspect it won't be finished before Christmas!!).

If it's just the general idea that is the same/similar then it might not be so bad. And you could always state that you started it before episode 'X' was shown - or in this case, before season 3/4 was shown let alone season 5 anyway!!! I've read fics that have stated that sort of thing before, and the similarities are often not as bad as the author feels they are!

So, don't panic just yet...and stay away from those spoilers!!