It seems like the most common suggestions are either A) STAB U-Turn, or B) STAB Crush Claw/Seed Flare type move that has a solid chance of lowering stats. So my thought is, why not combine them?
Lingering Shade
Type: Ghost
PP: 15
Power: 50
Accuracy: 100%
Effect: Switches the attacking Pokemon with a Pokemon of the Attacker's choice. Lowers Attack, Special Attack, Defense, or Special Defense of the defending Pokemon one stage, chosen randomly with equal probability. The stat lowered is revealed before the attacking player chooses which Pokemon to send in.
The typing can be either Ghost or Steel (with the suggested name Rust Rush, if Steel). The power can probably range from 40-80 depending on how much this was to be pushed and what the final effect is chosen as. The effect could be either as written, a guaranteed 1-Stage fall of a given stat (probably defense), or a 40-60% chance of a 2-stage fall of a given stat (again probably defense). I kind of like the idea as currently written, however, as it provides some interesting opportunities as to what you choose to switch in. If their Pokemon gets a defense lowered on the switch-in, you can send out a Pokemon designed to take advantage of this lowered defense. If they get an attacking stat lowered, you could take the opportunity to send in a stat-up Pokemon and start to set up.
It has the benefit of scouting for a switch with standard U-turn procedure, but has the added effect of enticing them to switch again immediately, revealing more of their team. Steel would give it no immune Pokemon, allowing this to be used on things like Blissey and Togekiss, but has overall weaker coverage.