Mundo can get an Ezreal-level change and be fine.
Nocturne can get a more useful Shield (blocks the next attack/ability, rather than just "Spell effect") OR his tether changed up a little, and focus on his "infinite pursuit" fantasy the Q and R provide. Visually I never saw him as a problem. Could also confirm whether or not Nocturne is a future Zed or related to his powers in any way - Likely thinking is not, since Nocturnes last known location.... was Demacia.
Shyvana is absolutely perfect please do not change her.
Voli beats Fiddle in one issue - Lore.
move his Passive to his R and then fill the blank with something that brings his whole kit together and he'd be fine, but his visuals and VO don't match the new and incredible lore.
Fiddle has an unhealthy kit, full of poiint-and-click CC and targeted silence, a really effective but basic passive, and can't do his intended job well anymore (AP Jungler) with the removal of Magus.
He needs help, he looks like he's from an original Playstation game, his VO is about 45 seconds long, his lore is ... questionable. Good, but lacking in substance.