So, I recently began watching the old fashion Teen Titans (not the new crap) and Slade screws with everyone. Just constantly screwing with them... is there a champion with that kind of attitude? Just screwing with everyone else to make them question themselves constantly?
Nocturne, possibly? Or Shaco?
Tahm Kench is also known to be quite mischievous, in a way that often costs other people either their sanity or their lives (or both).
Reworked Talon
That seems like it would be 
bread and butter, given what we know about her.
Additionally, given the one new scrap of characterization we got from her recent short lore story, 
seems like someone who'd play around with someone's head a lot too.
Literally Aurelion Sol
Taric makes me question myself about my sexuality, but aside from that, I don't know of any champion that does what you say.
LeBlanc or Swain would probably be closest to Slade's mentality in the current run of champions. At least, I assume, I don't think their lores have been officially updated yet.
Nunus counterjungling, even if you never gank you will tilt the enemy jungler so hard they'll go to lanes and feed your teammates because they have no gold or farm, or nunu bot and lulu laugh spam
Xerath insults a lot and make them question their power level, same for Aurelion Sol.
Tahm?
lux laugh is on par with jokers imo
