I've been thinking about this game for a long time and a few ideas that me and my friends have had is:
being able to have larger matches maybe 1v1-6v6 giving huge battles and crazy tactics that people can come up with.
unit grouping would be great because highlighting with that circle was a pain.
maybe adding a feature that you can have 3 extra guys in a game for each UNSC character that could take control of a spartan and communication would be key for this showing that they can either be supported or left to handle them self this would also make it slightly harder for people as they would have spartans that could be better or worse depending on the operator also taking out the A.I issues that will undoubtedly arise with the new programming we have seen with other games such as halo 4 and 5.
having larger armies would mean that a match could become more versatile and cause people to plan their army more in advance.
a specific game type for people that only want to rush as it is impossible to find a match where you wont get rushed any more and its hard to find a decent game where you either have to rush first or spend the first 5 minuets building up a counter rush army and defenses.
having a wider choice of units for both sides of the game being either the covies or humans to further the versititlity of a army such as being able to train specific sniper units or being able to customise your marine squads before a match such as a slower fire rate but more accurate by equiping them with dmr's or br's or having them as a assault squad with ar's and shotguns.
sabotaging buildings with spartan's being able to send a spartan in to a enemy building and having it blow up a minuet later and having upgrades for the interior of your bases to try to prevent such things happening and for the covies being able to send a spec ops elite squad in to a unsc base and doing the same thing.