You could do some user research by running surveys to understand pain points and use that to determine capabilities that would lower cognitive load. Prioritisation could be based on a number of things:
• What do your users want most based on user survey/ interviews
• What do your stakeholders want?
• Whats slowing down fast flow? Anything patterns emerging from post mortem?
In terms of measuring success, you have a number of options
• Adoption rate, your platform only adds value if people use it
• Perhaps measure like " Time for a developer to become productive" e.g how it takes to onboard a dev and get time to commit to production
• Lead time and frequency of deployment could be other measures that are important too
• Running another survey to check user sentiment is also very good too.
Have a look at SPACE metrics