Always units on ground, underwater, or in the air/space. Would be nice to have a underground unit carrier. Load up say 20 attack units. The carrier burrows through the planet and soon as it surfaces all the unit are ejected at once for a fast attack. Or units that burrow into the ground and wait to ambush any enemy units.
"We're being attacked!" "From where?!" "From EVERYWHERE!" Cue Leviathans rising out of the ground while the eerie glow of nanolathes pulse in the background.
Not sure if anyone else here played an RTS called Perimeter but it had one of the best underground units of all time... Essentially a large drill with a train of nukes behind it which burrowed under the ground faster and faster until it set off a series of explosions in a large line. It was perfect for breaking the bubble shields in that game.
That's the problem. Perimeter had shields and flat surface was important to build stuff and to gain energy, but in PA there will be no shields and surface is not a problem, so underground units won't have a niche to fill. They don't have anything viable to do. You may say they're good as defence-penetrating carriers, but we already have unit-cannon for that.
I agree - I'm all for different modes of travel. I always feel that land, sea, air is a little stale. I always loved Tiberian Sun for the subterranean vehicles. PA already has a new one though: orbital. Subterranean doesn't really seem necessary here.
The problem with subterran is always that its hard to show (to the player) and give good info about to make it fun compared to alternatives that fulfill the same role without overcomplicating battle awareness. Also, most subterran units are hard to balance as they get an inherent op advantage by design (can't be attacked while underground). Ofc if you make them attackable whats the difference between an invisible unit and a subterran one...