Temp sensor - are the canbus and obd sensors accessible when the car is turned off?
If not, or for older cars that don't have these, a separate sensor might be useful if one intended to monitor cabin temperature with some kind of smart-fan/sunroof/window combination.
I could see even a bluetooth proximity event that checked the temperature of the interior as you approached it and opens the windows to reduce temp before entry if it was really hot. Don't know if canbus would give you that or not.
And hence why (a battery) voltmeter is often a standalone instrument! (Apart from cheapness.)
FYI - Thy have released another vehicle here with automatic rain sensing wipers that cannot be un-auto'd (that can be lethal with bug splattered screens).
Similarly auto-dusk lighting, buy that only effects private eyes & spies.