Brian suggested, as part of an answer to the D&D Next/5e dilemma, that dungeons-and-dragons be changed to dnd-all-editions, and dungeons-and-dragons be used for 5e (since that’s how WotC’s branding it). That was quite unpopular, particularly since 6e will likely also be branded as Dungeons & Dragons without specifier, and then we have the same problem again.
But I think making dnd-all-editions and making dungeons-and-dragons a synonym of it has a lot of merit. It means anyone who tries to tag their question dungeons-and-dragons will have that change to dnd-all-editions, immediately making it clear what the tag actually means, and potentially saving us some fixing. It’s also consistent with the rest of the D&D tags, which is nice.