Not limited to large capacity, but to low-density applications where size isn't critical. Also anywhere that very cold weather is likely

So no wearables, phones, etc... But more stationary applications.

But solar mesh nodes (home or agricultural/industrial), off-grid pumps, power banks (camping style), home solar storage, lead acid car battery replacements, weather stations, electric toothbrushes, lights/lanterns, light tools like laser levels, toys and RC cars/boats, etc... Things that traditionally used alkaline batteries (often recently switching to Lithium for recharging) are often good applications.

Good Lithium-ion batteries have around 350 Wh/kg, so if you retrofit your devices, you'd be cutting out almost half the capacity, so at the moment it's not feasible yet.

Once the form factor is there and the loading voltage is the same (or the charging circuit can be replaced), I don't see why it shouldn't work

For high energy density lithium ion is still king.