Hello Russell,
Currently there is no way to make any of the fields on the native Overview tab required (sad face).
To get around this situation, if we need a field to be required, we create a Custom Field on our form and make it required. However, if you were looking to tap into the auto-notification process, etc. you may want to consider making an exception report to identify all Projects were the Sponsor field is blank. You could constrain your results to look at status so that you are really only looking are projects that are under review/planning; just to filter out any new or completed projects.
We have requested that Workfront provide some means to make fields on the overview tab required. I may have to add that to the new Idea Exchange; I don't know if that particular request ever made it to the new site.
