Solid state batteries. Prototypes work, but high-volume manufacturing doesn't work yet. The major battery manufacturers are all trying to get this to production.
Early versions will probably be expensive.
Maybe a 2x improvement in kwh/kg. Much less risk of fire or thermal runaway.
Charging in < 10 mins.
The one thing I'm curious about with solid state batteries is if there's a path towards incremental improvements in power density like we've seen with lithium batteries?
It would be unfortunate if we get solid state batteries that have the great features that you describe but they're limited to 2x or so power density. Twice the power density opens a lot of doors for technology improvements and innovation but it's still limiting for really cool things like humanoid robotics and large scale battery powered aircraft.
Somebody may come up with a new battery chemistry. There are many people trying. There are constraints other than energy density - charge rate, discharge rate, safety, lifetime, cooling, etc. Lithium-air batteries have an energy density which potentially approaches that of gasoline, but decades of work have not produced anything usable.[1]
There are, of course, small startups promising usable lithium-air batteries Real Soon Now.[2]
The big issues with hydrogen are volume and form factor. Hydrogen needs to be cryogenic or high pressure, and either work best with big spheroid-like tanks which don't naturally integrate into the wings where fuel is currently stored.
There are now a few large flow batteries. Here's one that's 400 megawatt-hours.[1] Round trip efficiency is poor and the installation is bulky, but storage is just tanks of liquid that are constantly recycled.
