I propose allowing posting new comments on any post I already have comments on, even if I drop below the rep threshold for the comment-privilege (50 rep) later.
And maybe also always allow responding to any ping (editors, dupe-hammer-wielders and mod-closers can be pinged even if they never commented).
Background: I recently made this comment on a post. Shortly thereafter, I put bounty on another one of my questions, which brought my reputation below 50. Then, when this follow-up was posted, I was unable to respond because of the requirement that I have at least 50 reputation to comment.