In our oranization, we do it in a this way even thought we are not doing Scrum at this point of time. We assess individual based on "Performance" and "Potential". "Performance" candidates are performing to the expectations of the job or role. "Potentials" are those who can go beyond their role and have out of box thinking. So, we promote "Potentials" and provide path for the "Performance" candidates to scale to "Potential". You can define the criteria for Performance and Potential based on your organization Goals. I am sure that your Scrum team has both Performance and Potential Team Members. I am not sure how much this helps you. But, it may be a good start.