There have been rumors that Disney is interested in bringing Yoda back in future Star Wars films. Why? Because kids love Yoda and he further ties the new trilogy with the previous 2 trilogies. Also, it would make sense that his Force Ghost would appear to our Jedi protagonists.
This rumor makes sense, as does the rumor of Hayden Christensen returning as Anakin Skywalker. Anakin has appeared in all of the previous movies and has just as much motivation to show up in the future as Yoda, being a Force Ghost himself. Who knew that George Lucas' unpopular idea to have Hayden play Ghost Anakin actually might pay off?
But if Yoda and Anakin return, then that would definitely mean Obi-Wan Kenobi must show up. Unfortunately, the original actor Alec Guinness has passed away so Ewan McGreggor would be perfect to play a Ghost Ben. And as a HUGE fan of the prequels, I would love to see Hayden and Ewan together for the first time since their brutal lightsaber battle in Episode III.
I would imagine that he's the most important piece of this puzzle. Without Obi-Wan, why would the other two show up? And he is the closest ally to Luke Skywalker.
This is only speculation, but its a great opportunity for the new Star Wars trilogy to carry on these legendary characters. And Ewan deserves a lot of credit for the success of the prequels. He was excellent in all three and deserves to be the linking factor between this trilogy and the last.