There's the strategic element of ground combat and there's also the visualisation of it.
If people are leaning towards the strategic element and not too fussed about the visuals, then I'm sure there's a way of creating some sort of mini-game, utilising the current ground combat engine, which could be engaging.
The first question is what's the point of ground combat and how does it benefit the wider game?
The obvious answer is to limit the destruction of districts, buildings and the population when assaulting a planet; I'd rather have all that stuff intact so I can quickly utilise the planet's economy and resources, once the invasion is over. A defender, on the other hand, has the choice of destroying key infrastructure so it doesn't fall into enemy hands, or perhaps retain it and try to fight off the invasion.
With that in mind, any mini-game could be built around these areas; you could even have an 'evacuation' protocol if you're the defender, where you're trying to get your population off the planet before the invaders land. Of course, if you're the invader, you're trying to stop the defenders from doing that; they're taking away your workers! But how do you stop ships in the void, if you don't want to destroy their population-cargo? There could be a dynamic for this, or perhaps you just have to stop them from taking off!
Population is one of the most valuable resources in Stellaris, so instantly the stakes are raised.
And of course, all of this would be planned out decades before an invasion begins, by spending resources, building units, buildings and ships, etc, etc.
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Anyway, there are interesting things that could be done with ground combat without going overboard on graphics or the like. There are loads of really simple, creative and fun boardgames and videogames out there that the devs could get ideas from.
I mean look at a boardgame like 'Blitzkrieg'; you basically fight WWII, across the whole globe, on one 25cmx25cm map, in 20 minutes. It sounds ridiculous, but it works really well
