This might be considered a duplicate idea as I’ve seen similar requests. However, they’ve been specific to certain needs and so, I’m submitting a general idea. 

It would be wonderful to be able to add calculation fields to a Report. This functionality would act in the same way calculation fields can be created in SFDC Reports or Rule Query Transformations. 

Example:

Our team records a large amount of detail for our product usage. Our Reports are build for certain Sales initiatives and require a unique calculation to be completed. Today, we have to create a calculation field on the Object in order to have it available on the Report. This causes a few challenges for us:

• The calculation is only used for a short time. This adds the maintenance steps of continually adding and removing calculation fields to the object. Would be much easier to maintain directly on the reports. Less effort and confusion.
• We hit challenges providing accurate calculations when the fields are grouped or filtered. 
• Field overload, we end up with many similar fields. This causes confusion while building the Reports.