Thinking out loud here:
A limited number of creep session play slots are available per region.
The level of the creep matches the level of the mobs in that region, but the creep may be "signature".
A player signs on as a creep and visits an NPC in Gramsfoot (ring over head if slots are available).
Player selects an open session play slot and becomes a level-appropriate creep in the selected region of the world.
Alternatively, the player could take over control of an existing NPC mob in the area.
The idea is to inject some intelligence and creativity without drastically altering the character of the area.
The plus side is the surprises and ingenuity of a few player controlled creeps.
The down sides are the work it would take to implement this and some players don't like PVP.