We already have custom notifications, but they are not on by default.
Pre-populating notification by default (but allowing them to be turned off) may give a better user experience.
Fridge light – a 10 or 15 minute warning about being left on - easy
Hot water heater – maybe 2 or 3 hours – either you have left a hot water tap running or you have a leak! In this case it still maybe trigger if the leak is small so simply an alert of unusual usage patterns may help.
The “ON” alert for some devices which cycle e.g. have a thermostat may mean something more like 90% on in a rolling time period.
Having some defaulted on alerts may be a better way to let people know about the functionality – “works by default” principle. What the default is would of course depend on the device type.