Lammy and Kinnock are the main two here who did not endorse Corbyn in 2015.
Surely the main one would be the one who stood against him, even if he did join his Shadow Cabinet.
Anyhow, my thoughts:
Diane Abbott - Nah, controversial and probably too past it by then.
Andy Burnham - Seems content with his current job, and not really viable after two lackluster attempts already.
David Lammy - Lammy as a future Labour leader is mostly a late noughties meme. Clive Lewis is much more likely.
Stephen Kinnock - Like Hilary Benn, will probably always be lower key than his father.
John McDonnell - Too past it and not particularly liked by the PLP (even for someone on the Hard Left)
Cat Smith - Possible, but Rayner and Long-Bailey are talked of more.
Cat Smith would probably be my choice.