I would love to see OT characters represented in GE, but I think the reason they didn't just make a Star Wars "themed" land that could include characters from different eras is that the original concept was for the land to be 100% immersive/interactive, with guests cosplaying and being a part of the story (kind of like the marketing for Galactic Starcruiser). To remain true to that concept, you can't suddenly have characters who are now dead walking around the land.
Of course, a lot of those elements fell by the wayside in the actual execution of the land. Also, what percentage of guests are committed enough to play acting the whole time instead of just tourists sight-seeing and riding rides?
Back in the day, I loved the Hyperspace Hoopla at the Star Wars Weekends. But think about how utterly ridiculous it is for Darth Vader, Stormtroopers, etc. doing the YMCA!!!! Even if they didn't go to that extreme, just having Darth Vader make an appearance would break the immersion on some level because it's an acknowledgment that you are visiting a Star Wars themed amusement park and not actually ON that alien planet and you are part of the saga (unless they announce that he's a cloned reincarnation that Palpatine and the darkside cult created).
It's kind of like how Pandora was (still is?) portrayed at AK... I remember on my first visit asking about gift cards in the gift shop and, without missing a beat, the CM told me I would need to "return to Earth" in order to find one.
So I don't think we'll see any OT or PT characters unless Disney shifts gears and abandons the concept that guests are "living out the story." That can only remain consistent if they keep GE "locked in time."
They could, technically, include Luke, Leia, Han, etc. by slightly adjusting the "date" of when the land is locked in time, but they would have to be the older versions of them if they want to maintain consistency to the timeline... unless Disney perfects a new holographic technology that would allow force ghosts to walk freely about the park (even better than "real" lightsabers). Also, you get into nitpicky details like the fact that Luke is still MIA until after Han is killed, so having both present creates a plot hole/inconsistency.
I think both approaches have their merits, but as someone who watched the original Star Wars in the theater as a kid in 1977, I think the trade-off would be worth it to include "all things Star Wars" in GE, especially since many of the interactive/immersive elements that were originally conceived have already been largely abandoned (I'm looking at you... Docking Bay 7 Menu!).