In Guild Wars you have a class, called the mesmer. His strength are Illusions and Trickery. Actually there are many thing which could be implemented for the Harlequins.
Psychic Power: Target an enemy infantry unit. Until the next Psychic Phase if the unit tries to move, to shoot, to charge or to fight in close combat roll a D6 for every model in the unit. On a 5+/6+ that model takes a mortal wound. The minds of the soldiers are confused and take friends for foes.
Stratagem: Set up an Marker (best is a Harlequins Model) within 12" of an enemy unit. Every action that unit does has to be made against that Marker. It is an Illusion that taunts the closest enemy unit to attack it. Can be activated before the enemy turn starts. Marker is removed at the end of enemy´s turn.
Stratagem: Fog of War. A friendly Harlequins unit and every other character model within 3" get a -1 to hit for enemy attacks in the fight phase. I always think they act a bit like the drunken master.
Stratagem: Void Blast: Let a friendly void weaver shoot with a void loaded charge. In the next charge phase that unit cannot fire overwatch. That is quiet cheesy.
Stratagem: Present from the other World. If a friendly Masque unit falls back or if an enemy unit falls back that turn. Roll a D6, on a 5+ that unit suffers 1/D3 Mortal wounds.
Just my 2 cents on that matter. Unfortunately I don´t know where to hand in those Ideas.
Furthermore I would like to see a Troup Master on a Jet Bike as much as an Shadowseer.
P.S.: Yet another cool Stratagem: If a friendly Masque unit falls back it leaves an Illusion behind so that the enemy unit is still in combat until the beginning of the players next turn.