For a federal charge, I don't see an obvious reason why such a plea deal would be illegal or unconstitutional, but I'm a little dubious that this could be done with state charges. But in any case, (a) Trump would never accept the acknowledgement of guilt that a plea deal would require, and (b) the Biden administration would never offer such a deal because it would foster the narrative that the prosecutions were purely political.
Exactly this. What more, it would set a terrible precedent that one can be a just awful criminal before, after, and during one's term in office, and you'll get off with a slap on the wrist and restrictions on being back in office as your punishment. Not exactly good for the rule of law.