There is some good news: the Rusted Pack File Manager has been updated just this week with the TW:W3 schemas. While there's no official CA Creation Kit or Workshop support, it's now possible to make and use some basic bug-fix mods.
One dude has already fixed the buggy script I was complaining about the other week that made the mono-god Chaos factions unable to remove negative corruption traits. I've started work on fixing up some of the more egregious oversights from moving from TW:W2 to W3; I'm almost done fixing all the Lord and Hero traits to bring them all back up to where they currently are in TW:W2 Mortal Empires (like undoing the nerfs to traits like Knowledgeable, Strong, etc. while also buffing the new faction-specific traits to get them up-to-par).
Unfortunately, without "official" support from Creative Assembly and the Steam Workshop, using these mods is kind of clunky. They need to be directly installed in the game's data directory as movie files, and have to be manually removed or edited anytime the game updates. Still, it's a start.