*My response is centered around lyrical content
I was (am) marginally more of a McCartney fan than Lennon (perhaps 52/48), but if I could turn back time, it would be super-interesting to knock ideas around with the mid-period / pre-Yoko John, due to his lyrical ingenuity--perhaps Van Dyke Parks and Donovan could sit in with us
Directly contradicting the above, a Carole King collaboration would be valuable in the sense of keep-it-simple-stupid: no need to add gratuitous complication to excavate depth of emotion; I would allow Jackson Browne to sit in with us
Follow up--which two singers (male & female) would I choose to sing the products of these cross-pollinations?
Annie Lennox & Carl Wilson