Could you look into Kathus , Twitch and Kassadin's ? Many of Karthus's and Twitch's splash arts still have their old models and not the models post Vu And Kassadin's just look horrible (Other then Cosmic ) Edit : Forgot Sion, his NEED updating
Don't forget the donger. 
I think they never specified any specific order for the splash art updates. So its entirely possible that those that you already mentioned could get their updated splash arts before the ones mentioned in the thread-name
Its not even like they have to do the splashs for one champ at a time,they could mix the order of the splash arts.
Though overall I think Karthus is actually not that much in need for new splash arts. Sure most of his skin splash arts are quite outdated,but his VU didn't change his design very much,so they still resemble his ingame-model rather well. Same goes pretty much for Nidalee.
