A wizard is never early, nor is he late. He reprises his most famous role precisely when he means to.
Talking to Graham Norton today on BBC Radio 2, McKellen responded to the host asking about the prospects of another actor playing Gandalf by saying, "What do you mean, another Gandalf?"
He went on to say, "I haven't said yes because I haven't been asked. But are you suggesting that someone else is going to play it? Gandalf is over 7000 years old, so I'm not too old."
Normally, I don't like to report on casting rumours like this -- actor wants job! is not exactly news, most of the time -- but this seems like a special occasion. McKellen could have any role he wants, with his prestige and talent, but he's at least a little bit intrigued by the idea of taking up the mantle of the wizened old wizard again.
Amazon, you should probably call him. Just saying. It'd be a good move.