Forest Map
As you most of the biomes for a military game, tropical, desert, snow, urban (I’m counting tunnels as that) etc, I think the next best map would be a temperate forest map with pine trees would be great.
I will talk more about what it would mean for each aspect of the game.

1. The map itself and game modes. Bases & P.O.I.’s : The map would be oval (you could place this in a valley) with the HQ’s being farthest from each other. The terrain would be a little hilly, similar to “Iceland” I would imagine it having 3 heavily fortified bases arranged in a semi-zigzag diagonal pattern from edge to edge, these would be connected by the main paved roads which should be about 2 tanks in width. Then you could have 2 or more outposts, these could be the tall towers from some national forest parks, that would be placed in the empty areas. These would be connected by smaller unpaved trails just large enough to move a tank. Trees: The rest of the area would be dense with trees. These would only be destroyed by high explosive, C4, rockets, etc, or aircraft crashing into them. This allows infantry to still use it as a cover, but the enemy could use an Apache or a tank to pressure them.

Vehicles:
An Apache and a transport helicopter would be at each HQ as well as some humvees and 4 wheelers. A tank would spawn at the closest heavy base to each HQ and a little bird could spawn in the middle heavy base.

Summary:
This map would push infantry and vehicles drivers to work together to protect each other as a helicopter can easily get over the trees and drop troops off but it would be exposed to rockets. The trees can also act as cover for aircraft. It would also allow for a lot of infantry vs infantry and vehicles vs vehicles combat.