Guild Wars 2

Player/Anet Dev. controlled bosses

This idea gives players the ability login to a server
to play as a boss enemy.

Example:
*Champions enemy players raiding a city
*Enemy player meta bosses

*Enemy ability: transform player into a enemy.
Transform enemy player is losing health fast. So he must
inflict damage on other players to absorb their health. After
the transformation wears off. The player returns to normal.

No thanks. This brings in a whole slew of balance issues and it would be classified as griefing. How could it possibly be a good idea to let players who are very familiar with the systems be boss creatures against brand new players for example?
They won’t implement this for the same reason that we don’t have open world PvP. If you want to play against other players go play PvP or WvW. PvE is meant to be strictly PvE and exists for players who don’t want their game ruined by some kitten.

You’d also have the problem that some people wouldn’t actually do anything as the boss besides run in and stand still so everyone can get free loot.