Yeah, sorry, I realize I didn't explain it very well.
So, currently a hero has to individually promote his proficiency in each of the combat types (melee, mounted, archery, siege, naval), which I call combat type promotions. He starts with a certain value and he can only achieve a certain predetermined maximum.
I propose that you eliminate this. Instead, each hero is assigned a value for each combat type. So, let's say we have a hero right now who starts with no combat type promotions, but can
potentially achieve A/D/A/D/D if you fully promote them. In this system, you would eliminate the ability to increase their proficiency in each combat type. In return, the Hero begins with A/D/A/D/D.
Then you add a separate promotion: lets call it Leadership. Leadership can have the same 5 levels (D, C, B, A, S). For each level of this promotion, the Hero gains two advantages: 1. an increase in the number of units they can lead, and 2. increase the bonus for each combat type promotion.
Let's look at this second advantage more closely. Right now every time you increase a combat type promotion (i.e. going from Melee D to Melee C), your legion troops get a larger bonus. What each level of Leadership would do is allow this bonus for every combat type. Thus, Leadership D would grant the D level bonus for melee/mounted/archery/siege/naval. However, this bonus would be limited by the Hero's predetermined proficiency in each combat type as described above. That is to say, the bonus legion members get can never exceed the proficiency level of the Hero for their combat type.
Let's try another example. Let's say we have this hero with the following starting values:
A: Melee
D: Mounted
B: Archery
D: Siege
/: Naval
So, what would happen at each level of Leadership?
Leadership D: It would be like he has Melee D, Mounted D, Archery D, and Siege D in the current system.
Leadership C: It would be like he has Melee C, Mounted D, Archery C, and Siege D in the current system.
Leadership B: It would be like he has Melee B, Mounted D, Archery B, and Siege D in the current system.
Leadership A: It would be like he has Melee A, Mounted D, Archery B, and Siege D in the current system.
Leadership S: It would be like he has Melee A, Mounted D, Archery B, and Siege D in the current system. (but he can command more units!)
Now, I thought this system may be too inflexible. I offered the possibility that you add a promotion that would increase the level of each combat type by 1. Thus, our Hero who started with A/D/A/D/D would instead become S/C/S/C/C. Powerful, yes, but you can limit the number of heroes who can get it, and also allow only one level of it.
If you wanted to allow combined arms, I feel like the current system, specifically the combat type promotion system, would not accommodate it well. It would take many promotions (and thus hero levels) to get proficient in different areas to allow effective combined arms. Right now, getting to Melee A and Mounted A is 6 promotions! This is not even counting the "bonus" hero promotions, which really give the flavor that I love about legions.
Indeed, I would like you to focus on these "bonus" promotions! Implementing my idea means that a hero need only focus on one promotion, Leadership, to increase the proficiency of his legion members. This allows him to focus on the "bonus" promotions that can really let him specialize...
I hope this was clear! If not, I will keep trying!