Infantry
To start I want to throw out a mix of ideas for infantry units. The infantry tab in game to me is pretty underdeveloped. It essentially has just been "spam Engineers and nothing else" for the entire part of the game. Part of this is the other units not have enough strengths compared to Engineers and there could be an entire discussion around that (my suggestion would be to move Engineers up another core slot in cost making them more expensive then rework heavy infantry to like actually be worthwhile with much better stats for their cost). Regardless I think it would be good to see more unique infantry get added among the various tabs for nations.
The most obvious example I can suggest are Panzergrenadiers as a highly mobile infantry unit. I imagine they would be expensive but would be able to move like 6 spaces and still attack making them highly effective at attacking. The USA could also probably get an armored infantry variant as they also were highly mechanized.
Another example could be like German Brandenburgers which could be a similar unit to the British commando. Loaded with unique traits like camouflage they would be great at sneaking around and attacking enemies out of the blue. Stuff like this would at least make the tab overall feel less barren and give players some more fun things to play with. It also could be cool to see Italian Decima troops which were also commando like units.
I also think adding upgraded versions to the various elite infantry in nations would be an interesting thing. Currently most elite infantry appears in 1941-42 and then kinda just ends up matching the 1943 roster of infantry for stats. I think adding like a Soviet Guards 44 or a US Rangers 44 that retains a notable power advantage to other units would make them feel a lot more like the elite units they represent by being a "oh dear god" unit that is actually scary.
Recon
The biggest thing for recon I think is a lack of infantry based recon. Recon is almost entirely just modestly armored vehicles that don't hold up well against tanks. That isn't the biggest issue inherently but a lack of recon that can sit in close terrain is something I think could be fixed. There is many things to pull from but a full on recon infantry could be neat, you could also do super light vehicles similar to the kradschutzen and make motorcycles/jeeps close terrain fighters, or maybe something I haven't thought of. Still recon like infantry sorta feels limited and this is a clear space you could further explore to make more interesting stuff for.
AA and AT
I think these two tabs are worth mentioning as they both are fairly diverse but suffer from being extremely linear in what is actually useful. To start AA I think suffers from being to niche right now. Notably the ground fire for most AA pieces are just super underwhelming and I want to suggest buffing the ground damage that smaller caliber AA does. You could even make like 20mm AA guns fdire against soft targets rather then hard. AA just doesn't have a use the skies are clear so figuring out how to make that would make it more worth considering.
Then AT is a tab that is flooded with models that are all essentially just small variations of the same unit with one having like slightly more soft attack while another has better defense. It's boring and a waste of the effort put in. I want to throw out ideas to go back and rework these similar units and give them more niches. Some ideas could be like Stugs getting a modest soft attack stat so they can damage soft targets a bit making them more well rounded compared to other TDs. Meanwhile something like the Hetzer could be super cheap in core slots but suffer from low ammo and like not be super stellar in it's stats (although with how cheap it is that can still be nice to have). I could go on and on but the main thing is just make this tab like less about "just use jagdpanthers or whatever has the highest hard attack" like it currently does. Make the options more interesting and it would vastly improve.
It also could be interesting to see like smaller units like AT infantry which would be useful for scenario makers. AT infantry could be great at ambushing tanks and damaging them but can't support like an AT gun. It would be nice to see among various other AT options.
Artillery
I think artillery is ok as it currently stands although there could be an expansion of the options to more specific types. Notably something like heavy duty mortars or infantry support guns could something interesting to see as cheap but cost effective artillery. Right now you are mostly going to use rocket artillery or 15cm/equivalents for your artillery work. Having cheaper options that specialize against infantry would be cool to see.
Air
I personally don't like how the air combat is set up in this game (units deal way to much damage in one go leading to annoying peakaboo gameplay). However, unless one were to make damage less burst heavy and more so like consistent that won't change. As such the only thing I would personally want to suggest are more heavy style fighters. Currently you have like 2-3 examples in game like the He 219 which trades it's fighter power for raw damage. These are interesting but there are so few comapred to what you could offer. Examples include like the German Ju88 which had a heavy figheter variant, the American P-38s and P-61s, and soviet Pe3 fighters. It would be cool to see more diversity in the planes and how you could set them up as currently it's very linear.
A similar thing can be said for bombers where you kinda just pick the one with the highest stats overall unless you need a fighter bomber. I wish there were like distinctions between say bombers and rocket planes. Bombers would specialize in attacking soft targets while rockets and cannon aircraft are better at killing tanks. As the theme of this post goes more options is always nice for a player.
Other Gameplay
The final thing are some ideas that could help create new scenarios and open up way more design space.
The supply dump is a model that already exists in game but it's essentially just blocking a hex. I strongly think this should be changed and the supply dump should have a trait that allows it to actually be a supply source. Notably my idea is that a supply dump will consume 1 point of itself to resupply a unit that can't trace supply from anywhere else. This opens up tons of scenario designs by placing a timer on the player as to them saving an encircled unit. For example think of something like a stalingrad scenarion. As the German player you have a supply dump with 50 strength in the pocket. With 10 troops in that pocket they will have enough supply to last for 5 turns before running out and start suffering from encirclement. Your goal is to break them out and open a corridor. However, you can't do this in 5 turns and have to use an airforce to escort planes to the pocket which if successful will add say 20 strength to the supply dump. This buys you time and let's you keep pushing towards the pocket. Within the current game this scenario is not really possible without some black magic trickery as supply spaces on the board pretty much cannot run out and even if set to do they don't tell you when it will happen. Having a unit do this is far easier to show off and makes it easier to design with. I also should mention that you could put this trait on things like a truck so it's a mobile supply source for a moving pocket or if we look to say the pacific this could be a way to supply troops doing island landings putting a time limit on you that gets worse if you keep shoving more and more troops onto the islands.
If anything were to be taken this is what I would want the most as it alone just opens up a ton of options for future design.
I also hope we can see many of the minor nations get fleshed out at some point. Some like Hungary and Romania are ok currently with a couple unique things and a decent roster but like as an example the commonwealth is essentially just a bunch of the same British infantry but with a different flag on their arm and that is it. I wish like Canada had tanks, and artillery, and AA, and etc. Not much can really be done with these nations in their current state which is sad to see.
For my final idea I want to also just see more stuff made for different nations. AO 1946 looks great for the US adding a lot of stuff that is cool to mess around with and could be incorparated into scenarios and what not. I would love to see more prototypes from different nations. Thinks like early war heavy tanks like the Soviet SMK or the British TOG II. I would also love to see some tanks on like a 1946 British and Soviet roster. What new stuff could we see if they were still active and then and what prototypes would be thrown into action at that point?
I know this is a long post and thanks if you read it all. Just some ideas I've had as I await AO 1946

There is a much potential left in this game and I hope we can see it refined more and more especially in tabs beyond just the tanks.