I think the time to have done it was in the 2009 film. That time has passed. As it was written in the script for Shatner to show up, would have been beautiful, and a genuine torch passing above and beyond just Nimoy on his own.
But now? Nah. It doesn't sound or feel organic at all. Anything that has been suggested is just an excuse to do the cameo, not to tell a really compelling story. I love the old cast. I always will. James T. Kirk circa the first two Trek films will always be my defining image of the character. But that time has come and gone, thirty years on, now.
Sure we have the alternate reality to play with, but that can be done just as fine without the call backs to contrast the "difference" as non fans won't know why those differences are special in the first place. It's better for the films, and indeed if a TV series follows off of the movies universe, that this new Star Trek lives on it's own. The cards have been shuffled, let's redeal them and enjoy the different results.
But that was just a cameo of him wishing Spock a happy birthday long after he was dead. We need him to be back alive.