I’m not sure that having spaces in tags is a good idea. I currently have tags on one line in a YAML header in each file, separated by spaces. So tags with spaces would mess that up.
I do as @Dor suggested for multi-word tags:
Hyphens between the words works and using multiple tags is more flexible:
#minecraft-building-inspiration
#minecraft, #building, #inspiration
In my mind, hyphens already solve this use-case quite well and, I think, better than spaces would.
On another note, I have created another feature request that would greatly enhance the use of multi-word tags, regardless of how the words are separated:
Fuzzy matching in tag entry autocompletion