again, the post above yours shows how it COULD fit, but that is only one idea. instead of expanding the cirlce menu, you could upgrade buildings like you do with your base, only it would be unit production buildings, and there would be more units when they are upgraded. (not always better units, but sometimes) example - Air Pad has the vulture and hornet, well if you wanted to add a pelican and a bomber of some kind it would go as follows.
Air pad level 1(origional) - hornet, vulture (pelican)
air pad level 2 - hornet, vulture, bomber
the hornet must be in all levels of the airpad because it is a mainline unit.
the vulture must be in all levels of the airpad because it is a uber unit.
also, the tank and marines and spartans follow the same rules and must always be available, however, things like cobras, wolverines, and flamethrowers could be replaced by things like, snipers or antiviecle soldiers(javalin) ect. i dont know what to replace a cobra or wolverine with,
In addition to your question im going to add something of my own, artilery should either be a unit trained in the depot, or a building that cost alot and takes a long time to build. ex: covanant mega turret on your base, but cost money to build, and fire.(amount based off of other facors for ballance. this could also be a mobile covanant mega turret, or other artilery pieace.
ex. unsc: 120 mm cannon. similar to the ww2 artilary, but futeristic. i will make a breif draw up of it.
stationed on a base. cost alot to build and run. no upgrades, 1 minute cooldown, needs teck 5.(late game only, duh)
mobile version - needs factory level 3, replaces cobra. cost 1000 supplys 5 pop. has infinite range. fire a shell that will hit a target, and destroy it(scarab might live, depending on damage, not tanks though)(the shell should be equal to 3 rounds from covanant mega turret) in addition to the shell, it also releases shapenal similar to canister shell, but stronger and maby bigger. (also, if it is found to be to weak for cost and maintaince, it leaves napalm on the ground for 15 secondes !) cost per shot is 100, time between shots is 1 minute. has light armor and high health, ie, can do alot of damage with very little, but takes a long time to heal. not mobile while firing. cannot fire while in the red zone.
upgrade 1 defenders: a squad of marines protects the artilery, getting the same bonus as they would from cover (4 man squad no matter what, 1 per corner)
upgrade 2 emp rounds (y ability) drops a special shell that damage the target, same as normal, but also leaves an emp generator where it hit (similar to disruption bomb, but the thing lasts for a whole minute and can be turned off midway by destroning the generator(it has a health bar and is not invulnerable) the y ability takes 3 minutes to charge.
see, its good, but very weak at the same time....
So long as there is a Moon,
Flowers will never die....
and neither will us Wolves.