The advantage to lithium batteries for low-power applications is their low self-discharge (10 years in many cases), meaning if you create a truly low-power design you can run it for a decade. Other primary cells will drain charge just sitting on the shelf meaning there’s a built-in expiration.
In regards to the thread topic - the hardware part can already be done cheaply and in a plug-and-play manner with an inexpensive microcontroller, a breakout board, and some cheap sensors. There’s no knowledge or soldering or circuit design necessary for people to create their own hardware right now (but it’s even cheaper if you do have those skills).
What we really need is some help with the code! If you’re up to it, a few of us are hacking at some code started up by @Charles_Schwer over in this thread. If you have some skill with this we sure would love a few extra hands hacking at the idea.
The ESP8266 is dirt cheap and super capable, and it’d make a great platform for adding DIY sensors and control to existing things in your home.