1) More upgrades for every unit, in Halo Wars 1 the wolverine and kodiak had some upgrades, but now there is nothing to upgrade on them and this goes for a few other units too. like the scorpion only has one upgrade. It would be nice to see new unit upgrades. AND add back units like the Elephant and Cobra.
2) More maps that are diverse in gameplay and looks, and FREE FOR ALL!!!!!!!! ADD rebles back to the empty base plots with their marines and turrets so its harder to expand
3) Possibly a new building for the UNCS to make at their bases, No idea what but it would be cool. The banished get a mega turret, maybe the UNSC should have a unique building too, to further separate gameplay style between the the two factions
*) an idea way way way out there, make a forerunner factions to fight and play as