My personal favorite additions are the Hero updates and Guild vs. Guild. Other games have this guild v. guild event and its always super fun. To expand on it my idea doesn't have to be restricted to Infected vs. Human guilds either. The H.O.P.E. could go up against Vendetta and so on.
Also with heroes having perks it would change the dynamic of the game and get rid of their uselessness. Plus having them as a required unit outside of your 7 units and 1 borrowed unit would mix things up. There would be so many possibilities to add with their perks. You could have really awesome ones that increase the squad max health, increase squad attack, and decrease squad cooldown or attack duration modifiers.