Having to play-test all the guides for the AWDS-mode of Custom Wars Tactics made me think about what could be done to fix old-style Advance Wars once customization becomes available. Here are a few things I think could be done to make a better AWDS-style game.
#1: Steal a Crap Ton of Stuff from AW4
To make a better AW3, you need most of the concepts from AW4. This means taking the damage formula, the units (which is to say modify the AW3 units to fit as possible), and bumping down the prices of any AW3 units that don't have AW4 equivalents. Also use the AW4 damage table as possible, with some variations I'll outline down below. The sandstorms in this game should also be used in place of AW3's sandstorms. This will stop defense from being ridiculous while still mattering. Also AW4 fog, because it's basically superior in every way.
Most importantly, get that experience system and give it a slight overhaul (4 levels instead of 3, all of them get defense, and the stat increase is always by 5 points). This will give the re-worked AW3 a much needed boost in the defense department without, again, making defense stupid-powerful.
As far as re-working the units, my thinking is just to alter the prices; not necessarily weakening them, though. For example, the Md Tank would come down to the 12k price of its AW4 equivalent, the Neotank would bump down to 16k, and the Megatank would become 22k. The B-Ship would become the AW4 version; it's just plain better, so why the hell not. Do the same with all naval units, keeping only the Black Boat untouched. Air units should come down in price where applicable, but keep their AW3 capabilities; Fighters were to damn weak. Make the Stealth the same price as the other two jets; might be halfway useful that way.
Lastly, use the AW4 terrain, including its comm. towers. Terrain variety is lacking in AW3, and the AW4 towers keep Javier from being ridiculous in the same way as AW3. (Javier would be changed to double the effects of towers; he basically does this in AW3 anyway, so it's not a drastic change.)
#2: Steal a Crap Ton of Stuff from AW2
AW2 did a lot of things right that were undone in AW3 for reasons I cannot fathom. First of all, the charging system. Testing has shown that, in practice, the charging system of AW2 really isn't that much slower than AW3's; the slowdown amounts to 3 days at most. However, it's still slow enough to not be stupid-fast like AW3's, and will give d2d abilities and (hopefully) COPs the emphasis they're really supposed to have.
AW2 is also the last game where rain and snow worked like they should, in my opinion, so that should be brought back. They can't have the tileset-changing effects, obviously, but other gameplay effects should be kept.
Lastly, several COs need to be rolled back to their AW2 stats and powers, partly for variety, partly for balance, and partly to survive the AW4 changes. The COs in question are Lash, Max, Sonja, Olaf, Drake, Eagle, Sami, Kanbei, and Sensei.
#3: Steal a Couple Things from AWBW
AWBW had its lab mechanic and a decent multiplayer version of Sturm, both of which should be snagged.
The End Product
Assuming no further balancing is attempted, the end result of this will leave us with all the playable options of AW3, and the better part of AW4's gameplay variety. It does sacrifice some of the strategy provided by AW4's COZs, but AW4 pretty much explored the extent of gameplay variety you can get from that, so much that one CO, Penny, had an AW3-style d2d. Nearly every unit should be usable with this setup, and SCOP spam shouldn't be quite as insane.