There is no need for any of them at all, ever tho man.
It's been 30 damn years, the universe is so freaking huge, we never, ever, ever needed any of the main characters to come back at all. They simply did it as a way to connect the dots so to speak, and open the new world to everyone, young and old, and then progress the story from there.
And I bet anything, in time, someday, we will see all of them, Luke, Leia, Han, Chewie, etc, on screen together, with new actors/actresses playing them, in new stories. Not a doubt in my mind.
Similar to what you see with the X-Men franchise, or the Star Trek franchise, A new Indiana Jones, or plenty of others.
Might be 5 years, could be 10 years, maybe 15, but in time, someday, we will see those characters as younger actors portrayed, some untold story from after Jedi, before the Force Awakens. Too much money to be made for that not to happen.
But first, we have to let the OG's come on screen again, and take a bow so to speak. Same as how Obi Wan and Yoda weren't central parts to the OG trilogy, but were a bigger part of the prequels.