There is massive scope to investigate the handling of the Muller report. Hilary Clinton had to testify for 11 hours (?) re Bengazi. Just imagine what they could do with this - getting Manafort in and every other low life who had a role in the early days of the Trump admin right through to Bill Barr and his blatant lying.
But it might not be that issue that stings him the most - the corruption was so blatant that almost everywhere they look they will find wrongdoing. They almost need to pick the legal fights - the clear and obvious from the fog of day to day grift.