One of the topics that interest me the most in the story are the Imperial legions, especially the ones we haven't heard anything about. I'll try to make a quick reminder of what we do know and please make sure to correct me if I'm wrong or if I missed something!
Ist - Led by Quintus van Cinna, who took his own life after realizing the Xth wasn't joining them. Fought against the IIIrd for the throne after the death of Varis.
IInd - Responsible for conquering Corvos. Were eventually overwhelmed by the beasts during the Final Days and some of the survivors fled to Garlemald.
IIIrd - Fought in the civil war with the Ist. Formerly led by Nerva yae Galvus, who was turned into a blasphemy after the defeat of Anima. Currently led by Virgillia Corculum.
IVth - Led by now deceased Basch van Gabranth*. Responsible for the conquest of the regions of Bozja, Zadnor and Dalmasca.
*Some people believe Gabranth might still be alive
Vth - Unknown, but it seems they were to join the Xth when aiding the Ist in Garlmeald during the events of Endwalker.
VIth - Led by now deceased Regula van Hydrus. They were responsible for exploring Azys Lla and finding the secrets to bind primals. In theory the rest of the VIth is still trapped in Azys Lla.
VIIth - Originally led by now deceased Nael van Darnus. Responsible for the meteor project and the seventh calamity. Later Valens van Varro resumes the weapons project during the Sorrow of Werlyt, but is ultimately defeated by Gaius.
VIIIth - Unknown
IXth - Unknown
Xth - Briefly mentioned as the main legion that would come to aid the Ist during the events of Endwalker.
XIth - Responsible for invading Nagxia, where they commited brutal atrocities and slaughtered many towns. The massacre of Dagluk, in Nagxia, was particularly brutal and denounced by Varis.
XIIth - Led by now deceased Zenos yae Galvus. Responsible for the dominion in the far East such as Doma, and also in Ala Mhigo.
XIIIth - Unknown
XIVth - Originally led by Gaius van Baelsar. Responsible for the Ultima weapon project and the dominion over Eorzea.
So, do you think we'll see one of the unknown legions in 7.0? Assuming we go to the New World or Meracydia I think it could happen. What could have happened to them after the defeat of Varis? Are there some rogue legions still?