I'm going to suggest something else, though adding some new ideas seems pretty pointless by now.
Maybe, if you were to get about say, over 1,500 points(?) you could drop in AI reinforcments. By this I mean drop in Marines (If Spartan) or a Grunt squad (If Elite). This way the Fireteam thing fom campaign could be utilized. Maybe you could get a certain amount of reinforcements the more credits you have. So like, at 500 points, you could drop in a two Marines/Grunts, or one Rocket Marine/one FRG Grunt, and the more money you have the better type of soldier you get, so at like 2,000 points you could get a few Elites with some vehicles, or a Warthog Transport with 2 Rocket Marines and 3 normal/Shotty Marines.
EDIT: And maybe since you're only fighting Elites/Spartans, your armor could be the custom color you have on it if Spartan, and if you were an Elite your armor would have the corresponding rank color. So if you were an Elite with the Minor armor, you would have the blue color.
Anyone wanna build on that? Point out some flaws?
[Edited on 04.10.2011 4:48 PM PDT]