There's many cases in which someone has a question about what site policy should be, people come up with suggestions, and one of them becomes highly popular and thus is treated as a rule. So far, everything's working to plan.
Sometimes, it eventually turns out that one of these rules has issues, and could likely be improved. There are a few possible ways of doing this, but none of them really seems to work:
Editing the existing, highly-upvoted rule into a new rule (which presumably is almost the same but with clarifications). Understandably, people are reluctant to do this, as it's unclear whether the upvotes for the old rule would still apply to the new rule. I've done this in cases where a new consensus contradicts an old one (editing the old consensus the minimum amount necessary to remove the conflict), but it seems unwise in cases where it's unclear whether a new consensus exists.
Posting a competing answer to the same question. The problem is, even if the answer is better, this is an old consensus, and thus an old question (thus is unlikely to get many eyes even though it's been bumped to the top of the active-on-meta list), and it's going to be hard for it to gain more votes than the existing consensus, even if it's widely considered to be better. (Exacerbating this is that a number of upvotes on the current consensus are likely to be locked in due to age.) Normally you could at least partially solve this problem with a bounty (and perhaps making a trivial edit to the existing answer to unlock votes on it), but Meta doesn't allow bounties.
Posting a new Meta post to suggest a change to the rules. This would be the best way to get attention to the change (as it's likely to show up on the "Hot Meta Posts" sidebar if the suggestion is at all well considered; the bar for getting there on PPCG is pretty low). However, it has two major downsides: it's arguably a duplicate (and may well be closed on that basis), and even if it can be asked in such a way that it's not a duplicate (such as by having a narrower scope that focuses on the individual problem), you can end up with two consensuses that contradict, and arguments over which set of rules apply.
(This actually happened in practice here, leading to a consensus contradicting a standard loophole; the situation eventually resolved itself, via mass downvoting of the standard loophole until it was under the threshold to count, but there was a contradiction leading to rules inclarity for several months; note that the standard loophole in question still has a positive score, and it was created when the site was much smaller and thus there were fewer upvotes to go round.)
Possibly there are other approaches too; the list above might not be exhaustive.
In cases where problems have arisen with an established consensus, and you want to make a suggestion for improving it, what's the best way to go about it?