Nixon was pardoned by Ford without a conviction.

And challenging presidential pardons is not a thing as pardons are a Constitutional power of the president.

What crime would he be prosecuted for?

I think it will be challenged because there’s a lot of controversy around Fauci’s funding of coronavirus research at the Wuhan Institute of Virology, that may come out under the new administration. On the other hand, it is possible that the new administration will view preemptive pardons as a useful tool for themselves.