There are people selling various battery plates to go from one 18V battery standard to another, comments around the web say they seem to work ok, but You can't charge the batteries though. They still seem pricey to me.
A cool 3DP project would be to design a series of parts to accept various batteries, to a universal plate, then another series of adapters to go from the universal plate to the various tools. Combine one of each side, wire it up, and you have your batteries to whatever tool. Any people interested in helping with this? I can start with a milwaukee M18 battery going to a universal plate. With a clever design maybe the same universal plate could be used on both sides... ?