I don't see this happening. The only players with a tag/different colored name currently are staff, so if they added a tag for players and allowed us to change the color of our names we may be confused as staff members.
As far as adding something that works like the Shiny Charm does, I doubt they would consider that for obvious reasons. Although I wouldn't mind having something like this, anything that increases shiny chances by this much is just bad for the game. They become easier to find for those that have completed the dex, and their value drops for everyone.
I do like this idea. A custom vanity exclusive for those that have completed the dex. Depending on what it would look like, it could be rewarding for players to own something that not many others have.