If I had to pick 3 things I would choose:
1.) Units that dont have upgrades should be given upgrades. All units should be given upgrades. Some of these can be anti-air, anti-vehicle, anti-air, shield, weapon enhancements, and so on. **** THIS IS MAJOR ISSUES FOR GODS SAKE ADD UPGRADES TO ALL UNITS - SIMPLE -
2.) The maps to be more interactive. Have buildings that can be destroyed, weather that can affect you, flood that can affect you, BASES like in halo 1 you had to destroy first, bunkers around the map, different maps, acid rain that affects infantry only, terrain that affects vehicles.
3.) If you ever do add a new leader stop with every leader having pretty much the same damn units except for two that you change with the leader of course. Think outside the box. Seems like all leaders the same except for a few tweaks. REALLY?? You cant come up with anything else? Would be neat to have a flying air hero - maybe super unit would be a flying fortress that releases small air untis. AWEEESOME!
4.) Firefight is cool but change it up. Gets boring. Have different aliens-beats-maps. Just trying to come up with stuff. haha