I think it would be good to have a separate module which clearly defines every new metric or KPI. These could be implemented as a function for each of these, which takes all relevant series as input arguments and returns the corresponding series or scalar, resp.

Then the reporting module provides a single function that takes the data frame received from this as input and creates a data frame with new columns just using these functions.

Similarly further KPIs or data frames which do not share the same time index can be derived from that new data frame.

What do you think @Mohammad-Tayyab-Frequenz?