I definitely agree with new emotes that you can buy!
Imagine what would happen if Anet made dances available like “gangnam style” and “hammer time”…
I think the easiest way for Anet to apply them would be adding them as a reward for an achievement, or just put them in the gem store. In that way players still need to do some effort in order to get them and it will keep people busy and playing.
It would also be cool if the actual songs would be played around that character when dancing, but I suppose that would be too difficult with all the copyrights etc. People playing the emote and posting a dance video on YouTube will be enough I guess.