Have you any thoughts on a tech tree?
How will it progress?
Sorry to say, the techtree needs to be balanced as well. And this might be the hardest job. Since you introduce a player to a new unit. The mission might depend on that particular unit. And the multiplayer techtree should be balanced as well. I think, the latter is even more important than the single player campaign. Because a new unit might actually be over powered for its first job. Just for the fun of it
It is how I introduced some of my units in the past. "o my, we don't have enough resources for dealing with the bunkers. Let's get 1 artillery unit, we know it is expensive, but that attack range of 13 is more than enough to protect it from wandering survivors".