A new battlefield would be interesting other than just the simple ground and air, and while the weather is a great mechanic and though it can be a decider, it rarely is. Therefore I propose something that covers over 70% of the Earth’s surface
Or expanding on that, the ocean, actual ships with ports that may have to be built, improving infrastructure and income while also enabling the building of ships and submarines.
And onto submarines and ships, subs would have ballast chambers where suspenendium chambers lie, and would be a very fun little thing to have, fluid mechanics with flooding in would require bolt-doors, like fire doors but stop the spread of water, and pumps that have to be operated by crew. Maybe a new type of boarder with a diving suit, slow, but able to board from anywhere on the map and go straight to the enemies ship! As well as marines who move slowly but can fight them in the water-filled areas of the ship, and when a breach happens A order to ‘suit up’, slowing the crew down by a large margin but making them invulnerable to fire. All lofty goals of course, but I really enjoy submarines and you would hit another sub-genre.
The submarine community!
Plus, I think actual ships with actual propellers needing to be light enough to float would be fun, with interesting new land-to-sea weapons if An attack happened on a port, with a small bit of land for buildings and the rest for ships. So there would be a downside to building a port, opening yourself up to sea-borne attack by foul cretins of the other nations!
Pretty much I thought it was a cool area the game could expand in, and I actually thought it out more than I should of at 3:30 AM waking from a nightmare about octopi.
Also, a new avenue for monsters! The Kraken, just a massive re-sprited, stronger, fleshcrackers. Could be a bit of fluff for where the mad geniuses got the idea.