Looking further to see who will be in there, so far we know only Tau and Genestealer Cults. Lets see what the Warhammer Community is telling us this morning.
via the Warhammer Community
https://www.warhammer-community.com/2020/01/28/a-war-for-hearts-and-mindsgw-homepage-post-2/
"The Greater Good is set in the Chalnath Expanse, where loyal Imperial worlds are torn between their faith in the Emperor and two other competing creeds. On many worlds across the region, Genestealer Cults rise up, and plans generations in the making come to fruition as the servants of the Star Children seek to throw off the yoke of their Imperial oppressors."
Who is in it.
The Greater Good focuses on the the Imperium – in particular the Astra Militarum – battling both the invading T’au Empire and the rising tide of Genestealer Cults. Commander Shadowsun leads the Fifth Sphere Expansion of the T’au Empire forces into the strategically vital Chalnath Expanse, bringing her mastery of the Kauyon way of war – the art of the patient hunter – to these embattled worlds.
What Happens?
"That would be telling! Let’s just say that The Greater Good shows war being waged in a few different ways. As well as open battle, both the T’au Empire and Genestealer Cults use their best wiles and surreptitious tactics to try to claim Imperial worlds for their own. In the book, you’ll see a planet tempted by the promises of the Greater Good – peace and prosperity within an empire that values every life. Certainly sounds appealing… Meanwhile, another world will see the insidious Pauper Princes cult rise up to overthrow their Imperial overlords – only to find that they face the T’au Empire as well."