It would be more immersive than just lugging along tons of "military hardware" to get the buff in ground combat. And it would it not require an overly complex minigame to at least bring in the idea of vehicles in planetary combat. Distant Worlds already does a fine job of ground combat without excessive complexity.
You get:
Infantry - your baseline combat unit.
Armor - more powerful than infantry (obviously) but also takes up twice as much space.
Special Forces - expensive, powerful infantry that attacks enemy defense units by slipping through their lines.
Defense Units - do nothing for attackers but, as the name implies, excel at planetary defense.
Something similar could be done here using the same combat interface & screens already present. A military base (facility) could convert military hardware (product) into armor and/or defense units, for example, or be used to convert regular marines into special forces units with time (and money).
Now you have something akin to Axis & Allies game play:
Infantry - 1 Attack, 1 Defense
Armor - 3 Attack, 1 Defense
Defense Units - 0 Attack, 5 Defense
Special Forces - 1 Attack, 1 Defense, but targets Defense Units as a priority with a strength 10 attack against that class of unit only.
Dice rolls as normal.
No extra map, no mini-game, no major mechanics overhauls. This is something that could fit into a mod. Just rock-paper-scissors and it would give your agents something new to do (i.e. recon, so you know what to bring to overcome your target's defenses). Plus, naturally, it gives the player more options on how they want to handle their own land-based military. Focus on defense bases? Go on the offensive with armor? Or just spew out marines like we do now and hope for the best?