Discourse supports one level of sub-categories, so we are not limited to flat structure, but could think about grouping stuff like this:
Apart from Categories, there are Tags which enable poster to provide additional metadata and give us browsable views such as: https://meta.discourse.org/tags/tagging
The nice thing about tags is UX: topic creator can create new tags and already existing ones are autocompleted.
Tags are disabled by default, but can be enabled in
I feel Tags could be useful for grouping cross-category discussions related to ideas such as "IoT", "meetups" or "pubsub" "js-ipfs", "go-ipfs" "ipld", etc.