With the market being flooded by low cost East Asian rechargeable batteries (i.e. AliExpress etc.) it would be nice to have a tester that could measure the actual capacity of a rechargeable battery. So it must be able to charge a battery until it's full, and then integrate its discharge current over time until it is at its minimum voltage.
Of course, I'd also like to know the charging efficiency, i.e. the ratio between the energy you can get out of a battery and the energy you have to put into it to charge it completely.
This would be a really nice-to-have in any electronics lab, so I'm sure other people will be interested in it as well!