Creating your own report is a good idea. Why do you feel this is not a good idea. Giving everything OOTB would be not the preferred way as it is kind of custom requirement and if it is OOTB it will be shipped for all the customer. This will make more no. of rules and decreased performance.
Yes, CASE created per year/month and the concern is there are more than one PYID (value) in the work tables. also we have to build something which can be run forall the divisions without any extra trainings.
Also I was thinking PEGA might have something OOTB to do case count for license purpose?