This is not really bad idea, i think that this should be polished and straightened up, but it has a potential...
I most of invasions are mostly slapping more troops on the ground, its kind of repetitive after while. On the other side adding planetary grid means that there are a lot of stuff to take into consideration, and i mean a lot, what if planet is tidal-locked? where is rear echelon/support when you are surrounded, are there terrain considerations? are there special structures like to take for example factories or fuel refineries? How hard would it be to manage such conquest? Combat width, orbital bombardment (tactican not strategic). How AI would handle it? How hard it would be to the game (strain on pc) Some of those features i do not see point in without the others, some i would imagine that are incredibly hard to code on database engine.
Can it be chunked up into smaller feature-pieces that works together (i.e. something that can be done and tested in finite amount of time, i.e. around 3months)? Also i think that Steven should take a look at this from technical side. I believe that we can argue about anything, and there is good point on both sides on the barricade, but in the end if Steven says "no, it wont work or will take 3 years to do i am done with this goodbye" then its more on the pointless side?