Well, it is based on the timeframe of the new (current) triology. That's why the big ride (the battle escape ride codenamed "Alcatraz") has the First Order as the antagonists. Disney is going for full immersion with the land and that includes having consistency for the storytelling elements.
We don't really know if there will be M&Gs in the land, but presumably there will be as that's a Disney staple. It so, the most likely choices are Kylo Ren, Chewbacca and BB-8 -- like they currently have in Launch Bay -- but I guess they could consider C-3PO, R2-D2, Rey, Captain Phasma, or even old man Luke if they wanted and still be consistent with the timeframe of the land.
As for Vader or other earlier period characters... as mentioned, the current Jedi Training conceit if that these are Force induced illusions, not the "real" characters, so you could still have a similar training with a similar premise and have Vader or Maul or so on. But we still have never heard of a similar type of concept going to exist in SW:GE. I would hope they have some smaller attractions like that present though.
Getting back to the timeframe of the land, my hope is that when/if they build a second Star Wars land in WDW -- you know it will happen eventually -- that they may the setting of that land as being the original trilogy and can use the "real" Vader, Yoda, Obi-Wan, etc..