
In order for a rollup summary field to work, there needs to be a hierarchy to roll up. Cases, Issues, Risks do not have a concept of a hierarchy. Currently the best way to average out a field for cases is to utilize a report. You can also create a custom field and workflow rules that will keep a running total of a custom value, do the same for a running total of the count of records, and then have a formulaic field that divides the total by the number of records for an average. It will require multiple rules and I would recommend working with your PS consultant to get this configured.