I felt that the campaign forced me to a play a certain way for each mission. I'd like to be able to build all units in the campaign, and not restricted to a few. I think that would greatly improve replayability.
This is exactly how I feel.
Throughout the campaign, I couldn't help but get the feeling that each mission was designed to force the player into a different play style in preparation for multiplayer. (Which I will never play.) Whether it was rushing, defending waves early, researching tech levels, or capturing and defending points on the map, I always finished each mission with a sense of emptiness and lack of accomplishment.
There are many players out there who prefer to play at their own slower pace and research all tech and upgrades whilst exploring the entire map. This is almost never possible in multiplayer matches, and so the campaign is the only option for this play style. But alas, the design of each mission made it impossible to achieve a sense of fulfilment.
I also felt that the goals in some missions were very much the same as the original Halo Wars.
For example, in Halo Wars, there was a mission where the player had to protect and deploy Rhinos at strategic locations around the map in order to simultaneously fire on a Covenant shield. In Halo Wars 2, the Rhinos were replaced with Kodiaks, and the shield replaced with turrets.
The overall goal in the game was also similar: follow the enemy ever deeper into the installation to stop them gaining control.