This might be spoiler heavy, so read at your own risk.
One thing I hope they touch on more is that there is actually 3 factions, represented by the three colors you see ingame: blue, red and purple. I really hope they dive into the purple faction more. Because in the end, despite the endless red swarm, and the couple of docile blue legions, you also have the heavily mutating purple which play some role. After that, I think it's always nice to dive into the politics of the survivors: terrorists, rebels, vigilantes, surviving pockets of humanity, police forces, pro-con groups for legions, religions, etc. The first game touches those quite a bit and I hope they continue, as human survival usually has tons of sides to it like this.