First thing that come to mind as being much better in a non Team Preview format is Zoroark, which I think gets bumped from "lol" to "actually pretty legit." It's more or less a nonentity in standard BW play because it's really hard to surprise your opponent with a known Zoroark, right? Hazards really limit your ability to disguise Zoroark effectively as other mons - if Volcarona takes 12.5% from Stealth Rock, well, that ain't Volcarona - and you have to bring it in on something that a) Zoroark actually wants to be in against and b) isn't so bad for what you're disguising it as that it gives away the game. That places a lot of constraints on teambuilding and (paradoxically, for something that emphasizes mindgames and playing your opponent like Illusion does) forces you to play in a fairly rigid fashion until Illusion is busted. And it's not like Zoroark is a great mon on its own. It's alright - it could definitely be worse, anyway - but it doesn't really do anything especially well. It's just sort of a mediocre offensive mon that's too frail to set up easily, too slow to be suitable for aggressive play against offense, and not hard-hitting enough to do much against bulkier teams without setting up.
Zoroark gets a new lease on life in a blind format, though. It's gotten a reputation as a lame early-hype mon a la DP Electivire, and that's completely accurate, but there actually was some reasoning to Zoroark's early hype, at least. Before people understood that team preview was a thing, Illusion sounded goddamn terrifying, and in blind BW Zoroark is gonna be the monster people thought it'd be way back when. (Even if you can't run it with Swimming Goggles Starmie.)
Remember: Zoroark is bad because your opponent knows it's coming and can play around that knowledge. Blind BW lets you keep Zoroark in your pocket, though, and that's a gamechanger. It'd be a hell of a scout, for starters - since your opponent doesn't know you're running Zoroark as long as your team composition doesn't make it obvious, you can use to extract information about their response to other mons on your team and U-turn out. And since you aren't announcing Zoroark, you can even use it to scout against things it doesn't necessarily wanna fight, per se - there's immense value in (ex:) bringing in your "Ferrothorn," seeing what the opponent swaps in in response, and U-turning out the same turn their switchin shows up.
It'd still be very reliant on hazard and weather control - and on preserving whatever it's disguising itself as so status/HP indicators don't betray you - but it's a whole lot better, anyway, and what it does (information! information! information!) is really, really valuable in blind BW, which is a matchup-heavy limited-information format.
It also makes a solid anti-lead with Taunt and strongish attacks, or a formidable midgame hole-puncher with a usable speed tier, STAB priority, and the ability to come in, set up, and start firing off attacks when the opponent isn't really prepared to deal with that. It's also got a surprisingly decent set of "weird" utility options that would probably actually become kinda cute when you pair them with Illusion - I could see Trick and Foul Play both being pretty legit on a team that benefited from the support they'd provide.