Radon Monitor

Offer a wif connected Radon Monitor, it could be expanded to also monitor various air quality factors. You would need to be able to set alerts, graph over various time scales, export data, integrate with other IoT devices, user accessible API.

You can modify a open close sensor to create the leak detector.
Leak detector leads into the vac tube on the radon system should do the trick.

When the radon system poops out, the water in the vac tube goes level, triggering the “leak detector” sending an closed notification letting you know there is an issue.

Haven’t given thought to the rest however.