Teasing new content. Being teased with upcomming content and speculating about it. These are half the fun of new content in a game.
One way that hasn’t been tried yet, but which could work well the way GoW is set up, would be to have a mysterious stranger show up in-game. And there are two ways this could happen:
The hard way
A short quest chain with exactly that: the player helping some mysterious stranger who will then reveal themselves as the epic of a new kingdom. I refer to this as the hard way as it requires writing an extra story.
The not so hard way
Adding a troop as a weekly reward before its kingdom has been released.
This way the teasing would jump out of the announcement section straight into the game.