I had a similar thought although I was a bit crazy on my thread xD.

Although personally I think the Zenos-Elidibus battle would be in part 2 of 4.5 as a dungeon boss similar to how Illberd/Griffin was handled in Baelsar's Wall during the Far Edge of fate patches in Heavensward

As for how we'd become the warrior of darkness,I expect that the scene we saw in 5.0's trailer is from a solo instance fight at the end of 4.5 where Zodiark,making use of our severed connection with Hydaelyn,would temper us,allowing us to become the warrior of darkness and fend off Kiribu and the other Amdaporian light beasts coming to kill us.

And much like how we did with Hyadelyn in ARR where we collected the elemenal crystals from Eorzea's primals, we might need to collect crystals for Zodiark from Primals situated in Garlean territory.

Tho I wonder if the assault from the Amdaporian figures are the result of either Hydaelyn or Solus Zas Galvus's actions.