Shaco, to me, is about tricking the enemy, being a constant thorn in their side, frustrating them at every turn, being a nuisance, and, best of all, putting the fear into the enemy team.
That might give people the impression that Shaco should be toxic but I don't think he has to be. I think the fantasy of Shaco (what I outlined above) can be fulfilled without necessarily giving him toxic or frustrating elements to his kit.
For example, take Shaco's Q, Deceive. It's been the source of frustration amongst Shaco's opponents for the longest time, because invisibility is pretty hard to deal with (Yes, I know we can ward, but that's something external to Shaco. Just because you can build / buy the appropriate counter-items doesn't mean that a champion's kit has fair counterplay, which is the issue with Shaco.) If Shaco's Q puff of smoke was better looking, had a more polished VFX, and Riot played up his evil laugh upon disappearing - making it audible in a large range - Shaco would become much more fair to play against, while still spooking / mindgaming the enemy due to these FX. It would be harder for him to gank, but he could afford to be more powerful - either by having more damage or by up'ing the invis duration.
Another issue with Shaco is his Jack-in-the-Box. It's a great ability, wonderfully thematic and mechanically appropriate - almost. The Boxes have very little innate counterplay (Again, yes, you can buy an item to counter them. That doesn't mean they are inherently fair or fun to play against.) If Riot made Shaco's Boxes not spring right away, if they have a count-down timer like Jhin's traps, then Shaco becomes much less frustrating to play against because his enemies are no longer stumbling into hard CC and getting gibbed by him (which is his current play pattern: wait for a lone enemy to run face-first into a Box, jump on them and right-click during the CC and win.) Likewise, Shaco's Boxes need an audio SFX. Music or something that clues the enemy in to their location. The effect of this would be that enemies know there are Boxes around, but don't know exactly where, creating confusion and paranoia.
Visually, I don't know what would change necessarily. I think he could do with less armor, but I like the black-red symmetry of his suit. I also think they could play up the 'two' aspect of Shaco's kit, similar to how Jhin has the narrative reason for his obsession with the number 4 carry into his gameplay mechanics for why a sniper has 4 shots.
I also made a post about Shaco a year ago, so you can read that too to see where I'm coming from:
http://orig03.deviantart.net/142c/f/2016/288/0/5/05f898b77cdf0377c92ae38aa94b0ac2-dal2pkg.png
