There has been suggestion for a temp/humidity sensor and another suggestion for a water leak detector, but I am proposing a 3-in-1 sensor:
Pipes bursting due to freezing can cost thousands in damage.
Water leaks (like a washing machine hose failure) can also cause thousands in damage.
The addition of a humidity sensor would also help make sure basements don’t grow mold and mildew that can ruin items in storage. And with integration with smart switches, it can turn on a dehumidifier when needed.
So a long-life (minimum of 6 months) sensor that can do all three would be very universal and helpful.