I suggest that users be able to comment on questions where they already have comments. This is obviously an edge case, since it requires that either a user posts a comment and then drops below the rep threshold, or that a moderator converts an answer to a comment. It is the second case I mainly want to address. Usually, or at least theoretically, when a moderator turns an answer into a comment it means that it was useful enough to keep around as a comment. Once a user has demonstrated that they can make useful comments on a question, it seems reasonable to allow them to make more comments on that question. This would be mainly useful for clarification questions, which can require some multi-step communication between the asker and the commenter.
This idea comes from this question of mine, where a user seems to be trying to post useful clarification questions, but the rep threshold means that either he can't get the clarification he may need to answer the question or moderators will keep turning his answers into comments (unlikely, since his current behavior is basically abusive).
It should be easy to prevent abuse of this, since there would be very few questions where this could happen at all, and it could be stopped just by clearing that user's comments on the question where abuse is happening.