Take this rumor with a grain of salt, you should. Cosmic Book News is reporting that Yoda will appear to Luke as a Force ghost in Star Wars Episode VII to warn him about "a rising unbalance in the force and the need to preserve the Jedi." That sounds like a pretty standard rumor...but then it gets bigger.
There is also talk amongst production that would give a nod to the previous Jedi characters in the form of holographic pictures in the new Jedi Temple.Such a background sequence would certainly be an easy way of incorporating saga alumni who are interested in returning to the franchise, which several of them are.
It's said it wouldn't be anything major, but would be in a background sequence featuring holograms of Yoda, Alec McGuiness as Obi-Wan Kenobi, Liam Neeson as Qui-Gon Jinn, Samuel L. Jackson as Mace Windu as well as others.
3/9 - Oscar Isaac
3/14 - Star Wars: Return of the Jedi Special Edition (1997)
3/16 - Alan Tudyk