Having HomePods as an option for home theatre surround sound would be cool, but there's a lot of logic involved there. A stereo pair is easy. For true surround (not simulated surround) you would need at least 5 HomePods. Then, you have to figure out what device is going to do the processing for this setup. Is that logic going to live in the HomePods, or in a device like AppleTV?
For surround, not only do you have to decode the audio stream from it's Dobly, or DTS, or other encoding, but then you would need to calculate which HomePod to send the appropriate one of 5 different signals to, and you could not get it wrong. The complexity would also increase for a 7.1 system.
If a stereo pair (left and right) get confused as to which one is left or right, it doesn't really matter, the sound stage does not collapse. If you accidentally send center channel vocals in a surround setup to a rear speaker instead of the front center, then your movie experience gets completely destroyed, from a listening perspective.
Another thing to consider would be support for a subwoofer, which Apple currently does not make, and is kind of important in a home theatre setup. And cost is another factor. Five HomePods (not minis) would cost $1500. You can buy complete entry level home theatre surround systems, including sub, for less than that. The HomePod idea is an interesting one, but at this moment I don't think there is a value proposition there.