An obvious one. Should be easy. Ready source of energy.
For Smart Radiator Thermometers obviate the need to replace batteries by using the heat from the radiators to recharge the batteries. Use standard rechargeable batteries so that there are no issues in those parts of the world where the radiators are used infrequently.
It's not impossible, but it has some serious drawbacks. One being I expect it would make some form of noise, another the thermostat being bigger. Calling it easy is underestimating the complexity of changing a low temperature heat source to electricity within a small package.
This suggestion has been already made. Please vote for it here. This way we can have a clear overview of the number of votes for each suggestion.
Your tado° team