This is something I've posted a little of here and there over the past year or so, but I figured I may as well make a dedicated thread for it.
Basically I envision something that visually would be similar to Advance Wars, but with more depth to the gameplay, with things like armor facings, unit ranges, units getting affected by statuses (ie freezing or starving), and other similar stuff. I think the biggest fundamental differences with Advance Wars would be in the more core things, though, like having an "action points" system similar to Classic X-Com or Valkyria Chronicles, instead of the more simplified move-shoot 2-action system most turn based games have.
Another core idea that I'd like to experiment with is the idea of units being capable of being combined into larger groups. While the actual scale of the game would likely have each in-game "unit" as being roughly platoon sized (so a single infantry unit on the map would actually be about 40 men), several units (likely up to 4, since that number fits nicely onto a single terrain tile) could be combined into a larger group of units that would operate as a single unit, but have all the advantages of all the units in the group. For example, maybe you combine 2 infantry units and 2 tank units into a group, where the tanks being in front confers a defensive bonus to the infantry in back, while also limiting to the mobility of the overall group to that of the infantry. While this could increase firepower it could also be a disadvantageous in the case that the group can be targeted as a whole, making artillery barrages deadlier in this case.
Speaking of artillery, another idea I'd like to play with is artillery less as a kind of long-range sniper as in most strategy games, and more of a deterrent and force multiplier, where single artillery units can attack multiple spaces simultaneously, making them useful in softening up enemy forces before an attack, or for creating a kind of "barrage barrier" that damages units that pass through it, making artillery units useful in both attack and defensive roles. Might also be cool to differentiate between different types of artillery, with howitzers having long range but being poor against entrenched units, while mortars have a shorter range but are better suited for destroying fortified units. Experimenting with things like smoke and chemical weapons could be cool, too.
I'd also like to expand more on the environment having a greater role in the game, where it not only affects unit movement, camouflage and defensiveness, but can also be altered, so things like forests and cities could also be flattened if enough artillery is dropped on them. It would also make for a cool aesthetic, with the battlefield slowly turning into a grayish-brown mess after enough conflict has occurred.
I would prefer the game (at least, the single player campaign) operate somewhat similarly to a roguelike rather than a conventional strategy game campaign, in that you progress through the conquest of the enemy nation as a series of semi-randomized battles, where your troops and equipment is "purchased" along the way with funds you acquire. This would also present the opportunity for a larger arsenal of units that wouldn't need to be meticulously balanced; something I like is the idea of patterning each faction's unit list in a somewhat realistic manner, where early-game units might be primitive WWI-style tanks, however while these would eventually become outdated with the introduction of superior technology, the old tanks could at least stay somewhat relevant in late game by introducing upgunned/uparmored versions of those units, or variants converted to become tank destroyers or self-propelled guns. In-game this could function as a kind of upgrade system.
Anyway, since I've mostly just created artwork for the game, I'll use this thread to dump what I have so far.