As far as I'm concerned the series stopped being interesting after the second film. I understand that First Class was better than people expected, but I'm still not motivated to see it or even a sequel to it. Though I'm one of the only people on earth who actually liked Superman Returns, so my opinion obviously cannot be trusted.
Reading more, however, the idea of there being an alternate modern-day continuity is...compelling. I don't know how Last Stand would even be relevant in that case, however - if the continuity splits with Senator Kelly's death in the first film, the second and third X-Men films would have never happened. You could conceivably make this movie without referencing either of the X-Men sequels. Though you're probably still stuck with all the plot holes created by the Wolverine film. That...that was such a wasted opportunity.
Reading more, however, the idea of there being an alternate modern-day continuity is...compelling. I don't know how Last Stand would even be relevant in that case, however - if the continuity splits with Senator Kelly's death in the first film, the second and third X-Men films would have never happened. You could conceivably make this movie without referencing either of the X-Men sequels. Though you're probably still stuck with all the plot holes created by the Wolverine film. That...that was such a wasted opportunity.