It'll be a submod if I ever get around to releasing it. The implementation for end-users should be fairly simple, adding entire lines into the .txt files or recompiling their source files as I instruct. It's a very very simple concept really. Just to give you a hint, we'll be editing troops, adding some dialog options and adding some scripted events in triggers.
The hard part (actually tedious) is on my end. However it should be worth it especially for what it can do. Because NPC lords also use the system, it'll really change how you play the game, making it a bit more challenging. You might end up fighting a lord that uses pure cavalry, for example, which can be quite annoying for some army compositions. Other lords will be using archers with spearmen to protect them, and others will still maintain a seemingly random mix.
This is all controlled by their personality, which I think should influence their philosophies on war. There's also always a little bit of random variance of course, because I don't want it to become monotonous. A lord that usually uses pure cavalry might sometimes enlist a bunch of musketeers to complement the cavalry for example, just to spice things up.
I also plan on making changes to AI behavior, either included with this or as a submod on its own. It should complement this troop congruency submod very well. Because we know that the NPC lords with only one troop type (infantry/archer/cavalry) will have them all lined up in a ridiculously long line that's just ridiculous. Did I mention how ridiculous it is? The plan will be so that if the lord is using a purist/congruent army type then they'll try to divide it into multiple smaller units. So you should hopefully be able to see lords using multiple cavalry wedges, with one of them charging early so as to turn the flank for the other cavalry charges.
I had an alpha version of this in Floris and it was working alright for what few features I implemented. I'll likely keep it fairly simple though, primarily focusing on volley fire for NPC lords and flanking cavalry units, with some other minor features like spearmen being the frontliners.