What do you mean by 'main films'? Do you mean just the first 3? Because at this point, there have been 3 'main films' and 4 (soon 5) 'offshoots'. The implied minority is now the majority. I think it's safer to call the X-Men films a collection, and not try to segregate them into 'the main 3, and then these other ones'.
Even your GIF shows Ellen Page, who played Kitty (a main-ish character) in both The Last Stand and Days of Future Past. Along with about 6 other characters which ties DOFP to the original trilogy, essentially making DOFP part of your 'main films'. But then DOFP is also set immediately after First Class, again sharing nearly the whole cast, so that should be included too. Then of course you've got Origins which has the scene at the end with Xavier which leads it nicely into X-Men 1.
I suppose the only real offshoot is The Wolverine, but even that has the scene with Jean Grey which ties it back to the earlier films.