Should Rod Rosenstein resign or be fired, Jeff Sessions could simply withdraw his recusal from the Mueller investigation, because the focus of the investigation has drastically shifted from its original parameters. That answers the question as to who would take his place. Besides, at this point there is no more conflict of interest than which has occurred under Rosenstein’s tutelage.
Originally, Sessions recused himself only to avoid the appearance of impropriety. Rosenstein, however, actually is directly linked to the investigation through the F.I.S.A. process as a primary signator of one of the warrants.
Simply put, Sessions would still run the D.O.J., and if he doesn’t withdraw his recusal, the third in line, Noel Francisco, would oversee the Mueller probe.