Ultima can be a channeled ability
My fantasy for BLM is that Ultima is a Level 80 channeled direct damage ability, growing in strength every second and rising to a monstrous climax while simultaneously giving a Haste buff to the raid that keeps increasing in percentage every second. YES!!!!! :D
But it wouldn't really fix the BLM issue :(
Even WITH Ultima as our Level 80 nuke, we still would be undesirable for a 8-man party. More DPS won't help, unless there is an enormous increase in personal DPS for Black Mage to the point where the meta changes. I feel that there are 3 possible solutions:
1) Increase party sizes
2) Nerf raid utility for other classes
3) Give a unique utility ability to BLM so that it has its own "Trick Attack"-level usefulness.
Option 3 is the easiest one to balance by far.