[Update]: Total War: Warhammer 3's release date has been pushed back to early 2022.
While its release date has not been set just yet, you can expect Total War: Warhammer 3 to arrive at some point this year. It'll bring with it four playable factions (Kislev, Cathay, Khorne and likely Tzeentch), a massive new map to conquer, a rework of siege battles, as well as a whole bunch of other additions and improvements.
If you're curious what some of these additions might look like, as well as what the Khorne and Kislev rosters will be made out of, you'll be happy to hear that Creative Assembly has now released the very first gameplay preview! You can check it out right below, along with the new in-game cinematic trailer. Have a peek:
It's worth mentioning that these Survival battles will not be replacing normal battles you're used to from the previous games. Instead, you can consider them as an occasional treat that helps make some of the quests and invasions feel more memorable and challenging. Whether each of the factions will use the same mechanics, and whether there will be different types of Survival battles, that I'm afraid nobody outside of Creative Assembly currently knows.
Still, I'd say it's an exciting addition and definitely something that has a great deal of potential. After all, not every Survival battle has to last an hour. Some could even be small 10-15 minute skirmishes that help break the monotony of the usual field battles. And after sieging the 50th castle in a row in Total War: Warhammer 2, that's something I'd certainly be up for!
Whatever the case may be, I'll make sure to let you know once Creative Assembly releases more details. Until then, you can learn a tiny bit more about Total War: Warhammer 3 over at official FAQ.