Having ground combat be more than just numbers and a roll of dice. It would be player controlled battles with variables based on the terrain of the planet and it's environment and suitability cost would have an impact on your troops performance.
I agree. And my suggestion is a hex based system with the hexes based on planet size. Yes, I know how cliche hex based ground combat is, but think of the possibilities in aurora. Some units would be faster (like mobile infantry, marines, combat engineers), while the bigger heavy units would be slower (like heavy assault, and low tech units), and maybe an in between speed (HQs, construction, replacement, etc). Titans the slowest of all but they have a huge range. Maybe even making sectors on the planet based on manufacturing/civilian/military structures amount (all randomly assigned each planet on generation/making colony, and then the sectors grow as you add) as well as strategic locations (government buildings, power plants, etc). PDCs would also have a static location on the surface (either placed automatically or player placed on building/ordering) and PDC weapons could give support to nearby units that are engaged in combat. I'm 50/50 on weather or not you should be able to stack PDCs on the same hex or have a hard/soft cap based on the hex "type"(civilian, industrial, military, unused, etc) . Bombardment would add hazards to the battlefield, like zones that are uncrossable except by special units (and/or Titans) for a good while or damage any unspecialized unit that crosses. And when I say a hex system, I don't mean something small (like Stardrive size of a dozen zones across), I mean planets Earth sized should be around 100+ hexes in diameter (larger planets have a lot more, smaller planets with less). Asteroids would only be like 5-10 hexes and the "strategic" point would be any PDCs, industrial/mining sectors, and/or the entrance sector to any underground installations. On that note actually, asteroids with underground facilities could have a second hex layer to represent the underground just to keep them interesting (and combat engineers and marines would get a bonus because of similarities to ships/PDCs). It could also be made so you need sensor coverage of the planet with your own ships/satellites/installations to see enemy units. Units could also have their own sensor range for when there are no ships/installations that can see the enemy (with some units having longer range and some a lot shorter, with Titan having a really short range). PDCs would always be visible after you seen them once because they are static objects. These changes would also add a reason to add ranged support units like artillery battalions and such that would have similar mechanics as Titans, but a shorter range/damage but would be faster. But of course, the damage, speed, and range would be affected by research (mid-late game heavies as fast/faster as early game "light" units, mid-late game artillers longer range than first Titan, etc).
The units would be represented by those rectangle codes (difefrent units would have different codes, maybe the player could change them like how they can change flags). Titans by a robot shaped symbol. And PDCs by a bunker shape. Units/Titans/PDCs on the same hex would stack, and you would get a breakdown by clicking on the hex (like you can on the system view).
Of course, instead of the ground unit tab for all this, there would be a separate window. The current ground unit tabs would largely stay, but modified to not have the "initiate combat with population".
Another note is that this would be quite interesting with multi-nation games and I have no idea how that would be set up. Obviously there could be a border system where it shows the borders of the different nations' areas (the border itself could tell you the diplomatic status with them. Dashed white line for neutral/cold war, solid/dashed green for friendly, solid blue for ally, and solid red for enemy/hostile).
On the note of colors, hexes with active combat would be highlighted by an Orange color or have stripes going through.