I have noticed that the more we see of Halo Wars 2, the more it becomes obvious that it is being viewed as part of the Reclaimer Saga and setting up Halo 6. They didn’t want it to be a self contained story. After I just watched the SDCC panel youtube vid, they seemed to come pretty close to saying that the Prometheans and Flood will feature in the game. So in terms of advancing the plot and Halo Universe it looks like the game will address several really important plot points.
The fate of the “Covenant” as Atriox creates this new group. Possibly opposed to or allied to Cortana depending on how things play out.
Cortana taking the Ark/a Halo Array and her efforts to control the Galaxy.
Possibly even the return of the Flood
Now this might explain some of their plot decisions in Halo 5. For example, the game ends on a big cliffhanger as Cortana starts her assault on the galaxy. Well, they would have been fully aware that in a year (until HW2 got delayed to Feb) they would be continuing that story. They also would have been reluctant to advance the plot past that as Cortana would immediately go for the rings and that was slated for HW2.
Why the Storm Covenant gets such a bad deal. They must have intended to switch gears and focus on a Brute centric and new Covenant in the Banished. So they kill off Jul and have the Storm Covenant ostensibly destroyed.
This is of course just assuming that future campaign expansions are just on the Ark and don’t further increase the scope of the games. ie Atriox taking Covenant worlds back from Cortana. Cortana fighting a Flood outbreak. Cortana fighting to takeover Earth etc etc.
I mean even some of these things would be MAJOR plot points that easily eclipse a lot of what happens in Halo 5. I mean all that happens is Jul dies and Cortana tries to kidnap Chief before a cliffhanger. So do you think that pushing some of these into Halo Wars 2 impacted Halo 5?