Snail Games' new Age of Wushu (also known as Age of Wulin in Europe and its native China) is apparently going to allow you to do just that.
MMO Culture has the details, and the new boss sidekick system basically involves random boss mobs that ask players for help. If a player agrees, he will spawn next to the boss mob, essentially becoming one of its minions for an upcoming fight. The source article notes that Age of Wushu lacks grandiose fantasy elements, so the boss (and you as its minion) will still be human as opposed to a variation on the gargantuan crab/dragon/spider creature you may be expecting.
Another point of note is that the minion player will temporarily relinquish control of his character, and the avatar will fight alongside the boss AI against player characters (and it will provide substantial rewards if the human party is defeated). Finally, MMO Culture says that player decisions relative to the boss sidekick system will affect the world storyline for every player on a given server. We don't know how just yet, but we'll certainly keep our eyes on this curious new mechanic.