Type-Rey
Retired
It will be the final boss, only not in FGO., but so should being an "Alien God". Also I've had this "theory" for like the entirety of Part 2, it's based on nothing but a desire to have "TYPE-Moon" as the final boss
Arc's profile is pretty telling.
She first has to deal with Altrouge and then , most likely has to battle to either to get rid of CM all toghether somehow or just completely take over it.
It being the final boss in FGO still doesn't make sense from the old lore point of view where the Planet and it are buddies who made a deal way back in the day to protect one another.