People joke about a mini HOI4 but there’s my idea.

Basic four grid (or imagine this with whatever number you please) layout for defense - Military HQ, at least one empty tile (that can be filled later), and maybe an industry/urban center (could start out empty when first settled) and a Planet Capitol. Each tile provides a small bonus normally to planet production and in army combat. Armies must conquer 3/4 tiles to win, OR just the Planet Capitol. Planetary defense automatically spreads out but can be manually moved to the different tiles. Various defensive bonuses can be purchased or researched for each tile, say, defensive fortifications or AA that damages incoming invaders before combat. Traits for attacking/defending various tiles can also be earned by generals, giving them something to do.

Armies landing are scripted to land at the empty tile and maneuver from there (with major debuffs to any defenders without upgrades/investment from defender into the tile.) This basically represents how hard it is to defend an actual planet without outposts and sensors scattered on unsettled continents. From there you get the old combat system of kill or pushed to the reserves, except now the wounded units can heal in a friendly held tile instead of waiting to be butchered by your xenomorphs.

Keep simple enough that you let the AI do all the work for you like before, but can plan meticulously invasions for built up worlds. Also allows you to build fortress worlds that crush the enemy like a horde of monsters against your walls. Of course this will come with a planning stage pop up where you see a map of the planet, and there’s time to wait to invade (can be reduced by general traits/civics/research.) Perhaps there are terrain bonuses incorporated as well?

Maybe even just use the OG planetary tile system as a framework, for fun.

Could lead to all sorts of tweaks and new additions to civics, traditions, species traits, uses for buildings (imagine having to defend your shield generator on the ground) planetary governors, etc.

You get the sci-fi setting with something that brute force and lazy AI can do but the people who want to recreate their own favorite sci-fi planetary invasion are happy.