Evidence of bribery, graft, or other forms of public corruption are not subject to presidential immunity
(else you resign & your VP pardons you)
All federal indictments of presidents or former presidents shall be heard by an inter-branch court which appeals directly to the SCOTUS on expedited basis
Then there's my idea of electing a prosecutor general (candidates chosen by canvass of career DOJ) & a defender general (canvass of federal public defenders) & Solicitor General (appointed by president), who choose an AG who does not report to the president, but that's for another day