Cass gets her gameplay update before Sion's.

first of all no, Sion isn't even a priority, and i've looked through all the season's patch notes and announcements no where did Riot even mention Sion being reworked.

So what if it takes until season 6? the thing is that several other elements of the game happen to be ready at their own pace, Cass' team is not the same as Sion's probably; Cass' just happens to be ready sooner.

If you throw all teams to work on Sion it's gonna delay it even more (conflict of opinions as different teams work differently) all we can do is wait, he's gonna be released when it's ready, nothing else.

Also, by Meddler's comments on Sion, he seems to be vastly different than the version we know today, so his rework will probably bring a lot of conflict as well and there's no need to rush that either.