Other anons have probably already come to this conclusion but I'm between appointments and don't have time to read the notables so here goes my theory:

Sessions had to resign as AG because he has been in the Senate for years and would have had to recuse himself from the prosecution of Senators he served with or whose misdeeds he witnessed or had prior knowledge of. This is why no Senator or current Congressman can be appointed AG. So no Lindsey Graham or Trey Gowdy, at least not yet.

Sessions will likely be appointed secretary of DHS, where he will clean out the deep state obstructionists and SES holdovers, including Kirstjen Nielsen, and proceed with securing the border, ending human trafficking, building the wall, and enforcing all existing immigration laws. As Secretary of DHS he will also be heavily involved in investigating election fraud, as a matter of national security.

Matthew Whitaker can remain as acting AG as long as needed, up to the statutory limit of 210 days. He has no recent ties to the Senate or House of Representatives and is therefore the perfect hatchet man, with no recusal necessary. After the Senate and House are cleaned and the dirty senators and congressmen indicted, he can be replaced by whoever POTUS wants, or nominated and confirmed on his own merit.