Zilean needs a VGU, so I severely doubt they'd consider remaking his splash to something new.
And the reason that the splashes were swapped to the Chinese ones was because of the splash art unification project. Prior to like 2015, each region had their own splashes, so when something new came, they had to make a splash and post it. The unification project basically removed this need by making is so that all splashes world wide were controlled by one entry. So instead of China, Korea, EUW, EUNE, OCE, and NA having to individually put up splashes, they made it so that when NA posted the splash, it updated it for ALL regions. China's splashes were chosen as placeholders for this because China is particularly strict about game imagery, so that way, they already knew those images were going to work for China and not require other approval. Additionally, China's server was also one of the more recent ones, so their splashes were ALSO the most reflective of how the champions of the time looked with all their updates and whatnot. This is why the majority of old splash arts were replaced with their Chinese counterparts, and also why they will not be reverted.