Keep in mind this is 100% for the fun aspect
A new mode that does a few things differently to increase replay-ability, chaos, and give the player more choices for defending taken territory. This wouldnt be an enormous change from whats currently available, but far enough that I think it would make for really interesting gameplay longterm and keep people playing for a long time while also giving more choices to the modding community if they felt like getting into it.
1) A new mode where division composition can still be decided as it is now, but remove the cap on how many of each unit can be brought in. Keep the reinforcement point system, so you still have a limit of how quickly you can bring units onto the field. (I know we can technically do this now with certain mods taking unit counts up to 999, but a lot of them go in and out of working properly)
2) Defensive implacements/structures. The other big change would be giving units themselves the ability to place/construct things like trenches, barbed wire, tank traps, mines, pill boxes, mg nests, etc. All infantry types could place trenches, but only engineers could build the more advanced emplacements. AT and AA guns could entrench themselves in whats already in the game currently. In later phases, engineers could have access to higher level emplacements like stone pillboxes or a stone emplacement around AT guns that give higher defense bonuses. Could even have a new "fortification supplies" new supply type/truck that brings in whats needed for larger projects. Infantry could have just enough supply to build one section of trench, engineers would have enough to build a few things but not to make it really get out of hand. But you could really build up a defensive position for someone to attack if you wanted to put the time/resources into it.
3) Have all units, when brought onto the map, start at base experience and gain it through combat action. Not sure if it would be possible, but adding a medical station buildable that injured units can retreat to and take a good amount of time to reinforce could reward saving units. Especially ones that have seen a lot of action so far and have built up experience.
Im sure a lot of people might think this would be ridiculous and maybe far away from what the game is intended to be, but I just had the idea of what might be a fun game mode that doesnt have to be taken quite as seriously. We all like watching massive fights. WWII was full of battles with one side digging in, knowing someone was coming. Pushing up to take a city or hill, then building up the infrastructure on it to defend it or using what the last guy built to now defend it from him. I had a ton of fun with the unlimited units mods i've found, and the breakthrough type with one side defending and one attacking was a blast. It got me thinking of a mode that would allow for more flexibility and making that type of gameplay ongoing, not just until you break though that first line of trenches and scattered emplacements but making it so people, with enough time invested, could build really impressive defensive positions that would be hard to take. And the ability to bring in a lot of units over the course of a game would make you able to assault those positions and not lose hope because you just lost the only group tigers you could bring in for the entire match. Thanks for reading this much if you got this far.