I wonder How come no one ever thiught about it before. Such an awesome idea not only for fun, it could be used for many purposes including a medical I.D., etc. The awesome thing about having a tag on your body is that there's no limits to what you can create.
Some people just stick to the animated Tattoo idea of it but going further in thought there's much more that can be done and many things to share. It can be something fun or something a bit more serious, the important thing is that is would not just be a tattoo, it will be something beyond that.
It Might sound or appear funny and obviously not everyone see this as a fun idea, who gives a fuck anyways, people always talk shit either you have it or not. For example, a RIP tattoo could take you to videos or slideshows of photos referring to that person you honor with the tattoo and also shows how much that person meant to you, etc.
Another option could be ID purposes.
One thing to keep in mind is to be able to share just want you want to share.
(Obviously the Tattoo pics in this post had a tag added as an example and to help those who have concentration difficulties can have an idea of whats being said.)
The difficult part about this whole tag creation process is to rely on an artist that can be accurate enough to pull it off, otherwise it will simply be a mess.