I also think its very important to give PR authors more control over their PRs metadata (for example through a tagging bot) and implement something like rfc 30 (which I started but then had to abondon because of a lack of time).
The main thing is that there should be a single tag (“needs review” or similar) that marks all commits that are immediately actionable for any reviewer. That action could well be “ask followup questions” (and change the tag to “needs info”), “ping other reviewer with domain knowledge” (and change the tag, maybe also “needs info”), request changes (“needs work”) etc.