Yes, it would be possible to do that. As you allude, it will raise a few questions as tags serve slightly different roles in Discourse and other platforms. It will need careful consideration.
This is related to the tagging question, but goes beyond it too. There are various layers to this. We could implement a Mastodon-specific approach to improving discovery. We could also implement a “normalised” ActivityPub approach to discovery. On the latter front see
But yes, we can discuss further what the short, medium and long term product goals are and I can develop an implementation strategy taking those various layers into account. I’ve gone ahead and raised this with the relevant ActivityPub folks: