My idea for a new ultimate was a new transformation that keeps him in Recon form but gives him a slow firing beam weapon with no range limit and no damage falloff that can pierce enemy bodies and shields but not headshot.
I also think Tank will end up a basic transformation but I do not feel a flight mode is appropriate for Bastion. I am also not a fan of any sort of ranged auto-targeting.
I think Bastion definitely needs a new ultimate and Tank mode would be a sufficient mobility buff if he had access to it more regularly than an ultimate.