Caitlyn's yordle traps don't let go after 2 seconds.
Heimerdinger's turrets run out of ammo.
Gangplank's ship can't reach the proving grounds with its cannons.
Talon's cut throat would probably be fatal.
Vlad would laugh at bleed effects.
Nunu would run out of snowballs in the first couple minutes of a fight.
Taric would be fabulous.

Brand immediately dies in the river.
Nocturne immediately dies after being hit by Leona's abilities.
Items slow you down and boots do not make you run faster.
Riven's ultimate takes 10 minutes to cast and breaks on the first strike.
Blinding shot makes you permanently blind since it pokes your eyes out.

morgana has bra straps

Also, Evelynn trips and falls a lot, because running in high heels? Yeah, we all know how that ends.

Brand would be immune to ignite and kill Maokai and Zyra in 1 shot.

having some poor voidling's head wouldn't improve your long jump