That's not the direction I'd want to go with it. Yeah MoO2 had tanks+battlebots, but that was a simple system that mostly gave a defensive bonus to empires who'd lost space combat. Very similar to planetary shielding, I'd say.
The existing system of collateral damage in return for efficiency is better. I don't want to manage 5 army types, I want to choose the type of war I'm aiming for. Will I sacrifice the planet's infrastructure for victory, equipping everyone with fusion rifles which punch holes in buildings? Or will my troops take the field as glorious liberators, shooting incapacitating shock/gel rounds? Suffering for their less lethal but also less lethal ammunition?
Well that was actually also part of my idea. I mean I know it's probably never going to happen, but having to buy from a few different army types instead of just the one we have currently, and giving them meaningful difference in terms of how they impact the invasions they're part of through precisely stuff like collateral damage and also stuff like advantage against certain units and terrain bonuses would go a long way to make ground combat a lot more meaningful. You could even bring back the unit attachments feature and make something relevant of it, it's more that the numbers need tweaking more than inventing an entirely new system, as far as I can tell at least.
I realise my idea probably sounds overly complex for most people's tastes but what I'm really just suggesting is a system that mixes something similar to the unit system of CK2 (multiple units with various performance adjustments and tactics that mostly work 'under the hood') with an enhanced mix-up version of the EU4 siege system combined with Stellaris archeological system.
I imagine it going something like this:
- Produce an army from whatever combined arms solution you have access to and feel is appropriate. As usual except the number of options is larger than 1 and unit stats will be relevant.
- Hire/Appoint a general to said army, as usual, except more unit types, tactics and terrain features means more possible talents that are relevant.
- Send army to invade planet as usual, except instead of a quickie-battle and then you're done Planetary Invasion is a semi-automated protracted campaign that rolls a lot of dice 'behind the screen' with modifier dependent on attacker vs. defender army composition, unit bonuses/maluses in regards to terrain and vs. specific unit types, planetary defenses and features and of course general vs. general level and talents. You can also choose/change what sort of campaign you're doing(scorched earth, genocide, civilian regard, etc) dependent on empire/ethics/policies/general talents/unit types present/etc.
- Conquering an entire planet will take a while! The Invasion rolls for progress once every so often (weekly, bi-weekly, monthly tick?) and depending on level of success or failure, the attacker conquers part of the planet and now controls, and units take attrition damage for the campaign + buildings, districts and populace suffer from collateral damage. If needed, attacker and defender can land more units on the planet to support the invasion or defense.
- As a planetary invasion conquers territory the invader takes control of the buildings and districts of the planet and gain access to their resource output + options for interacting with the populace conquered so far. This lets some empire types purge/assimilate/enslave/whatever pops in a war before ending the war and claiming planets. Should maybe be at a reduced rate as compared to the regular version and require some form of resource payment.
- Once a planet is 100% conquered it acts much like it does in the current game design, except as noted above, and except the fact that a military presence is required to keep invasion rate at 100%, prevent potential insurgencies and maintain any sort of interaction with the populace.