Back around March 12th a small rumor started from the MLPArena forum boards about the possible return of John de Lancie to MLP. A member by the name of Fluttersquid had attended the Toronto Comicon, and after the panel had gone up to JdL to get a picture of Discord signed by him. He laughed and started talking about the lack of females approaching him. She mentioned MLP to him and he gave out a little spoiler, namely about returning to the show. (Thread found here).
Since we didn’t have a second source outside a forum post, we didn’t bother to post about it, but it seems someone has recently uploaded JdL’s panel from the Comicon, where at one point (about 29:22) someone asks if he had thought about doing more voice over work, such as he did in MLP. His response is “I’ve done actually another MLP, which is coming out.”
It is most likely that JdL will reappear as a voice in Season 3, as way back when Season 2 first started he made the response on twitter of roughly saying that he wasn’t in anymore episodes, but if we wanted to hear him next season, to let Hasbro know. JdL’s entire panel (roughly an hour) can be found after the page break.