[Update]: Total War: Warhammer 3's Shadows of Change DLC is coming this August 31st with a much higher price tag.
While the details about the upcoming DLC featuring Tzeentch, Cathay and Kislev are still nonexistent, Creative Assembly has now revealed some of the changes and tweaks that will be accompanying it. Most notably, you can expect to see a variety of new landmarks to conquer, an improved tutorial for Immortal Empires, as well as additional difficulty options to mess around with.
First things first, the new tutorial will come in the form of simple missions that will guide you towards things like your first battle or first additional settlement, as well as offer advice about city building and similar concepts. While the initial system will be mostly devoid of flavor and purely focused on functionality, future updates will add more narrative and specific tutorials for each faction's unique mechanics.
As for the landmarks, it's pretty much exactly what you would expect. Creative Assembly is working on adding a bunch of unique new buildings scattered across the world, each of which will provide you with powerful bonuses once constructed. A fairly simple change, but having more potential goals scattered throughout the map is always a good thing, especially if the bonuses can really shake up your playstyle.
The final, and perhaps most important addition, will be a brand new Battle Difficulty slider. The new system will allow you to fine-tune the stat bonuses the AI recieves, as well as mess around with how smart they're going to be on the battlefield. So if you want, you can have your enemies work at full capacity but without any sort of cheaty nonsense that makes it so their infantry is always better than yours.
Once again this is a rather simple addition, but it does open up the doors for further customization. Who knows, maybe a few updates down the line we'll be able to tweak just about everything relating to difficulty and AI behavior, thus giving everyone a chance to create the perfect sandbox to mess around with.
Whatever the case may be, I'll make sure to let you know once the Total War: Warhammer 3 team releases more information. Until then, I'm afraid all we can do is sit and wait.