I love the idea of having to actually defend the last city on earth. I think it would add a lot more character because you actually are fighting for something at this point. Maybe you could even go down into the city and build relationships with the citizens. Then when these attacks happen you actually feel compelled to save these people because they trust in you and have faith in you, and if they die you have this sense of grief and you could continue fighting in their memory. Kinda like mass effect. If they do that they should also add aerial combat too. When the enemy ships come they could scramble the guardians to intercept the drop ships. I think that would be amazing.