I mean... it's a Square property. No reason they couldn't totally include a questline that takes you across the ages and unlocks several dungeons in each age, with a final Trial series that pits you against Lavos.
Lavos, phase 1, would be a boss fight against a massive opponent, cuz it's huge.
Lavos, phase 2, would be inside the lavos shell against its ultimate evolution, with the ages clashing against the boundaries of time while you kick the tar out of it.
Drops could include Magus' outfit and scythe for reaper, a Crono's outfit and the rainbow blade katana for SAM, Frog's outfit and the Masamune (II) for DRK, Lucca's outfit and raygun for MCH, Ayla's outfit (or a prehistoric variation for males) and Stone/Bronze/Iron fists for MNK, Marle's outfit and a unique bow modeled somewhat after her crossbow for BRD, and finally a Robo pet.
The outfits are obviously not tied to jobs, I just wrote it that way as they tie together.
But yeah, this would be AWESOME to have a 2-parter Trial Line vs Lavos and Final Evolution Lavos.