In principle many things, but you're mostly restricted by what you can meaningfully physically build with off the shelf parts. Prototyping with a bunch of wires on a bread board is one thing... Sticking that on your wall is another. And most off the shelf dev kits have very few useful sensors/actuators on board.

The cannonical thing is an environment sensor (so a temperature et al sensor). Unfortunately, often a larger HVAC system will integrate with sensors through some proprietary protocol (even Thread based systems like Nest don't necessarily work with just any thread-based sensor, but you could of course hack something with Home assistant).

One of the maintainers has been using it on the side for a plan monitoring system.

All of these require some sensors beyond just the dev kit, which are typically pretty bare bones.

You can build a two factor auth device with the nrf52840 dongle (OpenSK is built with Tock), which requires basically no extra peripherals.

I've used the nrf52840dk to control a garage door, which similarly only need a couple wires.