I'd probably ask about the Venus and Mars sessions that John was (allegedly?) planning to visit together with May Pang (in early 1975?) just before he got back together with Yoko, but didn't - just because I've been wondering what exactly happened there ever since I first read that story somewhere (not too long ago), and what exactly Paul knows/remembers about it.
Is this mentioned in any of the Paul biographies (none of which I've read), by the way?