DOCTOR Mundo- I'd love if he got a whole new voiceover that mirrors how he acts in his new bio & short story even more than he does in his current voiceover (Likely & preferably with his eventual VGU, though).
Kassadin- Not only does he have one of the most lacking voiceovers in the game, but some of his very few lines reference a mechanic that has since been removed from his kit (Silence).
Malphite- Now this guy's voiceover is just straight up outdated- He's got a few rock puns and the iconic "You will lose", but that's where it ends. He probably needs a VGU as well as an updated story to go along with that, though.
Skarner- I personally find the way our good ol' Bracken talks in his new story incredibly charming- He speaks in such a serious and tired manner but his words and phrases are beautifully poetic (at least that's how I see it): Softskins (humans), Dreamsong, Calmland, etc; It's lovely, IMO.
Miss Fortune- I want her to show at least some of her serious side (Which she clearly has in the Narrative now) in-game, too.
I loved her in Burning Tides and Shadow and Fortune and I really want her to hear her interact with other champions (namely Gangplank, Illaoi, Twisted Fate, Graves, Lucian), talk about how she could sometimes use Rafen's help (on the Rift), about her bloody past and about her great ship, the Syren, amongst many other things.
Still, there should be a balance between playful, giggly Miss Fortune and Sarah Fortune, Captain of the Syren and the most seriously vengeful woman on the Serpent Isles.