Let me begin by saying that I'm only theorymoning here. But what the poster above said got me interested and I decided to type up what I believe is an interesting set. It's a little gimmicky but I think it could be fun.
Gothitelle @ Dark Gem
Frisk Ability - Relaxed Nature
252 Hp/ 252 Def/ 4 Sp.Def
-Thief
-Clam Mind
-Shadow Ball
-Hidden Power - Fighting
Now, I realize that Shadow Tag is the vastly superior ability, but part of me is worried that the ability, not the Poke'mon who uses it will just be banned from OU play for some reason. I could be wrong. But anyways I took one look at Frisk and asked myself why. Clearly everything the Gamefreak gods have done to each of their Poke'mon is part of some grand design and in this case I feel obligated to try my best to make something out of Frisk.
Enter her moveset. No Trick or anything like that. Magic Room and Embargo, and, what's this? Thief? I realize she has a horrible attack stat and that the Dark Gem won't do much for her, but I really felt like she could be used as a Poke'mon that tries to maybe mess up an opponent's tank or hell, any Poke'mon that relies on it's item. The Dark Gem should be consumed just as you use Thief, powering it up ever so slightly and removing her held item, then allowing you to steal the opponents and use it for yourself. Which you know exactly what it is in case you didn't thanks to: bum da da da! That's right, Frisk! Ideally you'd want to steal a Leftovers or something similar given her bulk and EV spread (which isn't all that impossible given how common of an item it is) and even with a Relaxed nature she can reach 149 speed, enough to outrun Relaxed Swampert and some other major defensive threats. It also makes her an even better candidate for Trick Room teams.
The rest of her moveset is pretty straightforward. Clam Mind it up and then use Hidden Power Fighting and Shadow Ball to hit anything in the game for at least neutral damage.
Thoughts? Comments? Suggestions? I thought it was kind of a fun idea but again I haven't tested it. OH! -and I haven't exactly read through this entire thread so I apologize if something similar was already posted.