Bastions ult seems weak because the time it takes to shoot recover and the projectile to hit is slow and quite counterable
So lets look back at some of his old ults that i heard about on a yt video with jeff kaplan talking about bastions old ults
Shoot through walls
At first this seems really powerfull but lets go and change it slightly
Bastion can shoot through a max of 2 walls, each wall reduces damage by 25%
This can be great when paired with infrasite or hanzos sonic arrow. Making them 2 heroes more viable, even at lower tiers
Bastion flying
This is really powerful but again lets change it slightly
Bastion can fly but damage reduced by 20% and damage resistance is removed
Bastion mine
This could be could be good
Again probably need some counters
Like can be hacked, destroyed or even defense matrix.
My proposed ultimate
Mortar shell
Bastion hunkers down into the ground and 6 slightly controllable mortar shells get fired one after another
They can be controlled by less or more power and slightly to the left or right
Each shot does 300 damage
Bastion has a 50% damage resistance but can be hacked or emped to stop ultimate
Shields block damage
DM cant eat mortar
Shots can be destroyed
Each shot has 300 health and travel at 10m/s at a parabolic arc