Syndra, Jayce, Varus, Darius, Lulu, Evelynn.. These champions are older than Rengar, yet Rengar had his splash art updated. I know some champions still need their splash art updated yet are newer than Rengar, and I know some are in the process of being reworked, but those champions I've listed see some play here and there at least. What has happened to the waves of updating their splash arts?
Meh, Syndra and Jayce's are actually fine, IMO. I agree that Darius, Lulu, Evelynn and maybe Varus could use a splash update. Also, Skarner, Rammus, Kennen, Fiddle, Volibear, Warwick, Irelia, Xin, Swain and maybe Teemo, Fizz, Hecarim, Malz and Leona. :P
They will continue updating Champion Splashes but there are things that are more important. They are doing it besides Skin Splashes and their other Artist-Works for League of Legends and when the new Splashes are finished, they will update them. But it needs time
I would especially like to see a new Syndra splash, one where she is preferably being actiony, as is the new standard.
Just because a champion is older than another champion that got an updated splash doesn't mean they need to be changed too. At that point you're changing stuff just for the sake of it, rather than out of necessity. The only champion you listed who actually has a bad splash art is Evelynn and she won't be getting one until her eventual rework, which is likely a long way off. Why Rengar got one I'm not sure, because I thought his old one was fine anyway, but ... I'd rather they not change splashes that are already good just because they did that for one champion. People like splash arts, and don't want them changed to something they might like less, so changing splashes for champs like Syndra and Lulu is basically just risking pissing off a lot of people for no real gain.
Be cautious you could actually get what you wish for. Katarinas old splash was awesome until riot decided to gut it for this new garbage. Leave well enough alone, maybe they will not ruin more splashes.
DON'T YOU TOUCH MY SYNDRA SPLASH IT'S AMAZING ;-;
The rest... shoves towards Rito take them not this masterpiece pls D:
I like their current splash art.
Old splash art? 
They teased Warwick's new splash at last year's world. Don't know why they never release it.
They also teased Irelia's new splash a few months ago during the LCS Europe but also never released it.
