I've wondered if maybe it's possible to pack batteries with a relatively harmless material that undergoes a very energy-intensive phase change. Water's the obvious choice, but it tends to drain away rather than stick around long enough to turn to steam. Maybe there's a better material, or ways to design battery cooling systems so that a burning battery turns into a very efficient steam generator, and thus can be cooled adequately with much less water.

I've also wondered if maybe EV's should be designed with an easy-to-access coolant loop. For instance, maybe you have a fixture you can connect a garden hose to in order to pump cold water through channels in the battery modules for emergency cooling. (Most EVs have liquid cooling already, so this could be in addition to that, or maybe you hook into the existing system and the already-present coolant gets flushed out.)