Life is busy enough that it needs to be truly special or epic for me to remember to watch something before its available on my TV or computer. This movie certainly won't be that.
My take on this is that its basically an unrelated mutant movie set in one of the countless the X-Men universes. If I try and think of it as actually part of the X-men franchise it doesn't work for me either ;)
I can't actually remember legion... I've been meaning to rewatch it... So I can't comment there!