Instead of making him some random-ass demon or "ohh so mysterious"-character - as he is now - put him (or her, tbh we don't even really know that as he always wears a mask and his face is kinda disorted to tell for sure, and Shaco's voice has this "fakeness" to it, as if he was talking in-character while performing a play) with theGlade and the Fae.
For one we only have one champion connected with this rather mysterious part of Runeterra, for the other I can't think of any other champion who would fit just that well into their archetype. Let me explain: the Fae originate from English/Irish mythology, and the name describes all sort of faires, forest spirits, and so on. However: While not an exactly evil or hostile group, one has to be extremly careful when engaging with them. They are cheerful and playful - but they know no limits. Being used to a world of everchanging magic and chaos, while at the same time having a disorted view on life due to their immortality, makes them impossible to predict. Everything that "is fun" goes.
The more harmless part of this can be seen with Lulu, who transformed children into animals just for the fun of it. But in the end she never wanted to hurt someone, her idea of "playing" is that EVERYONE has fun for as long as possible. But now imagine someone with a more... "selfish" approach to the whole concept of playing. Someone who just wants all the fun himself. And someone who views their world as their playground, just as a typical fae would. Imagining that sort of personality shouldn't be hard, there are people like this after all in real life already.
"But Shaco is human! He's no crazy fairy?!" - good point, but there's one more aspect of fae mythology: changelings - at least I hope that's the correct translation for the german word "Wechselbalg" (literaly "Switched Brat"). You see, fae apparently had this weird tradition that they would swap a human infant or really small kid with one of their own kids which would take on the shape of the kid and then be raised by the human parents. Meanwhile the human child would either be raised or eaten in the Fairy Realm - that dependes mostly on the sort of story you read. Occasionally both, which must have made family meals really awkward if you ask me.
As to why fae do this? No clue. Some weird version of cultural exchange? An attempt to infiltrate human society? Maybe they need humans over in their world for some magic that only they can cast once thaught? No one knows.
It would make sense for Shaco to be a human who grew up in the Glade, and it barely matters if he got there as a small kid or if he stumbled into it at some point of his life like Lulu did. He'd be the totally opposite of her, yet also part of the same group. She's a cheerful yordle, and Shaco a human of... well, I just speculate and say "darker nature", who got completly twisted by the nature of the Glade and the powers he recieved from there.
Also, the other eason why this could be awesome is that we could give one of the darkest and most evil characters in the game a pair of pink-sparkling-fairy-wings and it would be 100% ok with the lore.
And as a final note: we already have a music piece called "Lulu's and Shaco's quirky encounter" - that would make a lote more sense if both of them were from the Glade, no?