I think there's a pretty long list of champions who need VGUs. Here are my picks for the most needed:

All 3 of these guys look atrocious, and their kits, while fun to play for some, are not as streamlined, modernized, or even viable in some cases.
Cho'Gath's entire gimmick is getting so much HP that it doesn't matter that his kit is weird and clunky. Plus he's HIDEOUS, absolutely, irrevocably hideous, and not in the "unknowable HP Lovecraft monster" hideous. He's hideous in the "this looks like a child's drawing that was modeled for the SNES or N64" way. He needs modernization to make him look formidable, and less like a joke.
Fiddlesticks has a mass of random abilities crammed together that somehow work, but his overall cohesion is non-existent. He could definitely use some polishing and some refining, and something to give his kit some uniqueness and synergy. I personally like the part of the passive where he becomes still after a set period (it has GREAT prospects for a VGU, to give him an actual scarecrow look when stationary), but I think the bonuses he gains from moving again could use some boosting and could be altered to fit a new kit.
Mordekaiser is supposed to be an AP bruiser, but requires health to do abilities - something that conflicts deeply with his design as he's got very little in terms of recovery in his kit. Furthermore, I don't think sacrificing HP really makes sense on an demi-lich* like Mordekaiser, since he's a soul in a suit of armor with untold necromantic powers. I think Morde needs to be mana based, and his kit needs more impressive abilities to give us the feeling that he's an actual undead warlord necromancer who steals souls and thirsts for conquest. I think if he gets a VGU, his Q should be the only thing that stays, as it's kinda unique to him, but everything else should go.
*A Demi-lich is a D&D lich who has transcended regular lichdom and has removed their soul from their body, and their spirit now travels about, usually from plane to plane. While their soul is removed from their body, their body decays, leaving behind the skeleton, which typically acts as their phylactery - this is almost exactly what Mordekaiser is.