Our friend at JediNews have a source saying:
Ian McDiarmid is set to return to the galaxy far, far away for a part in Episode VII.
The specifics of the role are unclear, but it seems likely that McDiarmid will be returning to his signature role of Palpatine. Whether as the original or a clone of, we are yet to discern.
Could the Emperor be returning? Could he be a Clone? Could McDiarmid be playing the role as a flashback?
SO MANY QUESTIONS!
Just remember, all of this is just a rumor until Lucasfilm confirms it.
Maybe as a holocron?
I like having him involved because it would be cool to keep Palpatine as the overarching villain in the whole saga, even with the ST. But it will be a fine line between having him involved and not undermining Vader’s sacrifice in ROTJ.
I could see him as a Sith Holocron like Kevin mentioned, or maybe even just a flashback sequence. I don’t think it will be a huge part. I don’t think any of the Big 3 should have huge parts either. We’re getting into weird territory with so many returning characters. They can’t all play large roles.