I wonder whether John Healey would stand.
I don't think he resigned in preparation but to the electorate he does present a lot of potential positives eg "sensible" (Starmer keeps highlighting himself as "sensible"), acts in the interests of the country rather than party, whilst a loyalist he also predates Starmer so has a track record.
After the Makerfield by-election I don't expect but would not be surprised to be hearing "Colleagues have persuaded me what the country needs now is ... so I've been persuaded to stand for party leader".
Unlike other resignations, Healey didn't start running an "sources within ..." anonymous briefing campaign leading up to his parting which I interpret as him resigning for the reasons he stated.