Again, this is how I envision it...
All the possibilities (some crossover in categories):
- Selecting/aiming at targets (where to hit enemy base)
- Choosing how to split available weapons among available targets
- Selecting what weapons to use
- Choosing the timing/order of weapons fired
- Steering missiles (micro-managing attack in progress)
- Aiming defenses (classic Missile Command style)
- Activating defenses (turning on defenses that can only operate for X seconds before requiring Y seconds of cooldown)
- Arranging structures (maximizing coverage area for each defensive structure)
- Selecting what to build (basic cost vs benefit)
- Picking the order in which to build structures (factories only and no defenses for the first part of the game, gambling the opponent won't hit you early?)
- Selecting where to build (see arranging structures)
- Balancing the allocation of limited space (offensive/defensive/resource split)
- Balancing the allocation of production units/credits (offensive/defensive/resource split)
- Making decisions regarding tech tree research
- Operating factories (manually harvesting resources ala Plants vs Zombies requiring you to click on the "suns")
Disclaimer: I played a tower defense game once six years ago for maybe an hour or two, so there's probably solutions in that genre that might work better.
I'll think of some special abilities later, right now I have to get back to work...