I've been messing around with Prankster Riolu. Although its previous niche of priority Roar is gone, priority Copycat can still be a potent weapon.
Riolu @ Focus Sash
Lonely, 252 Atk / 4 SpA / 252 Spe (0IVs in HP/Def/SpD)
Swords Dance
Reversal
Crunch
Copycat
One of the main problems with Focus Sash/Reversal in the past (e.g. Medicham) was the weakness to Choice Scarf and priority users like Scizor. Riolu might have low stats, but it does have something that other Pokémon lack, and that is a way to give its 200BP Reversal priority with Copycat (this has the added bonus of getting in two Reversals for the price of one on the turn you go down to 1HP).
Ideally, you want to come in on a slower Pokémon after one of your Pokémon faints, so you can get in a Swords Dance first - with a Swords Dance under its belt, only Ghosts and double resists can switch in on Reversal (standard Skarmory is easily 2HKOed).
You can also use the priority Copycat to do other fun things too (for example, Copycat-ed Outrage deals ~60% to Garchomp), but unless you've already used your Sash, Reversal is usually better.
Obviously you'll want to run it with Rapid Spin/Defog support (as with other non-lead Sash users), and try to eliminate opposing weather inducers (Tyranitar and Abomasnow aren't big fans of Reversal, so they have to come in before Riolu is at 1HP).
As you might expect, Ghost types are a big problem; however, if you predict the switch-in, you can hit it with a double Crunch (enough to kill most Ghosts). This leaves you vulnerable to a revenge kill by a faster Pokémon.
The set also relies on it getting attacked (virtually any attack will bring it down to 1HP due to Riolu's terrible defenses); if it gets statused, it's not going to be doing anything at all.
As a priority user, it's slightly more powerful than CB Talonflame (Jolly Talonflame has nearly identical attack to Lonely Riolu; Riolu's Reversal is ~11% stronger than a Banded Brave Bird), but can't do much outside of its primary attack, whereas Talonflame has Flare Blitz and U-Turn.
Like other Sash/Reversal sets, this probably isn't massively playable in OU, but I feel that it might have some use if you're feeling brave.