See this is tricky. Ewan is probably my favorite addition from the prequels. He did an excellent job. But, the thing is, Force Spirit Obi-Wan is established as Sir Alec Guiness. You may be able to get away with it with make-up and have Ewan try to emulate Guiness's voice even more. Perhaps you can make the image more faint, like Obi-Wan's time is almost over and it is time for him to move on, like the Zahn books.
Yoda, obviously, they could still do as he is CG and Frank Oz is still around. The Force Spirit that would make the most sense, and I know I will get a lot of flak for, is Anakin. His Spirit has been established as Hayden, and the Star Wars Saga is the Skywalker saga above anything. Hayden has gotten critical notice in other things so maybe with Abrams directing as oppose to Lucas, you can have a decent performance. but, something tells me Abrams wants to stay away from any prequel elements.
What I would like to see is a solo Obi-Wan movie starring Ewan that covers his time on Tatooine, dealing with Sandpeople and locals when necessary, his relation with Owen, communing with Qui-Gon, and taking steps to make sure Luke remains hidden. That would be suited for Ewan.