For example, Opportunities to certain Regions, or Contacts to Marketing Campaigns. This way, you don't need to create reports at all, and can instead go to the desired Object and see all of the related records. Or, if you do create reports, you will have more of your buckets freed up. Here are some use cases to check out that get around reporting buckets:
Sales Rep Reporting - Create categories for certain sales teams or geographical areas.
Lead Source Campaigns - Create more general categories like "live events" that encompass specific events.
Opportunity Products on Account Page - Categorize your products into groups like physical products, services, or if it's a bundle.
Opportunity Call Notes - Bucket your types of calls. For example demos, answering questions, or talking with decision makers.