Right now you have to wait until a pending invitation is accepted to designate someone's role as Owner or Billing. This requires multiple round-trips before a role can be changed: Invite > Wait > Check for acceptance > Wait > etc. > Re-assign Roll.
Allow pre-defined roles at the point of invitation would be particularly useful for the Billing role. We often assign this role to less technical stakeholders. It requires a lot of explanation and hand-holding to tell them to accept the invite, but that they can't change/update credit card information until we go back in and modify their role.