While I would hardly call myself an EU4 expert, and I have never played a competitive multiplayer game of EU4, I still think these ideas, while well-intentioned, still leave much to be desired. Let's take that separate combat width and flanking range idea.
Let's say we have a combat width of 10 and flanking range of 2.
My understanding from this discussion is, under the current meta, once nations can afford to fill out their front lines, you will end up simply with 10 units of infantry because this is the most cost-effective strategy (cavalry essentially lose their flanking bonus, and the extra damage cavalry will do is not worth the cost). So what happens under the new system? Once nations can afford to fill out their front lines, you will end up with 10 infantry and 4 cavalry (2 on each flank).
What exactly has changed here? Sure, cavalry are being used now, which is nice, but the reason they are being used is the same reason you fill out your armies to maximum combat width - to not do so gives your enemy an undue advantage with flanking. You are basically just telling the players the following: Your combat width is 10 with infantry, 14 if you add a few cavalry. There is still no strategy here. Everyone will still run the same army composition.
What if you instead had a different system, something like thus:
Use the current system of combat width and flanking range. However, for each unit of cavalry (adjusted for casualties, of course) you have above the enemy, you receive some benefits. Some possible ideas:
1, Flat combat bonuses to the entire army.
2, Bonus pips to maneuver, possibly allowing effective maneuver to exceed 6, or possibly allowing an army without a general to negate a river crossing penalty.
3, A new mechanic called 'Initiative.' Each unit of cavalry you have over the enemy, possibly modified by leader maneuver, adds to your initiative. Initiative might do any number of things for you, some examples: chance of doing double damage on a combat round; +x to the dice roll; etc.
The advantage of my idea is that it creates some sense of meaningful choice for the player. Cavalry is expensive, so running around with armies comprised of maximum cavalry is generally not the best idea. However, if you need to break a well-entrenched enemy army, then a high-cavalry army might give you the buffs you need to win: a high effective maneuver means you negate that pesky river crossing, even though the enemy has a 6-maneuver general; the flat combat bonuses and/or initiative mechanic gives you the combat edge you need to overcome the terrain penalty.
The argument of 'well, everyone will run around with maximum cavalry, then!' is probably not going to be the case. Cavalry is already better than infantry in combat ability, but you don't see everyone running around with cavalry doom stacks. Rather, the player now has a strategic decision to make. Do I forgo cavalry in this army to save costs, but leave it vulnerable to an 'easy initiative bonus' if the enemy army brings a few cavalry? How many cavalry-centric armies will I make, and how will I position them?
This is a strategy game, and if you can make a decision more strategic rather than 'everyone does it because this is the most efficient way to play,' then that is a win in my opinion.
EDIT:
After some more thinking, I think there is some merit to having the effective maneuver of an army be simply the product,
[Army maneuver] = [# of cavalry (adjusted for casualties)] * [general maneuver].
Since maneuver is only compared to negate crossing penalties, fractional effective maneuver is okay. I think this makes some sense, and locks you out of maneuver if you do not use cavalry. Further, while having a 5 or 6 maneuver general, combined with even just a few units of cavalry, would essentially make the crossing penalty insurmountable if your enemy has a low-maneuver general, I think this is okay. The crossing penalty is not an end-all penalty, and the opposing army can overcome it by bringing a general with an extra shock and fire pip. This would also allow a player to manage his RNG to an extent, substituting extra cavalry in lieu of a high-maneuver general if high maneuver is necessary (e.g., for a naval invasion).
