Here are some of my suggestions for a Warband-style mod:
Chambers
I believe they worked in WB if you were in the beginning part of a readying animation as the enemy's weapon was hitting your body, and if you're facing towards them; it was simple but effective; BL changes this by making it based on weapon collision instead, which might sound logical on paper, but in practice the functionality is underwhelming, and it can ironically result in more unrealistic-looking lucky chambers happening at illogical angles
Swing arcs
Try to make it so that attacks don't get stuck behind you; just reimplementing Warband's animations straight-up would be a decent upgrade; holding a left swing and moving forward in BL looks awful in particular; the current swing arcs aren't suitable for a broader range of environments (especially closed areas), which might face further problems in future custom-made maps (but are a problem even in TW's maps)
Stances
This technically might involve skill, but only in the same way that juggling while riding a unicycle and balancing an umbrella on your nose is skilful; I think it's horrible for groupfighting; it feels like an unnecessary, inelegent feature; and it's funny that TW's defenders were once saying "yeah it's a bad idea but it has zero effect", yet various effects have since come to light
Directional shields
For melee it's okay, but the shield should more reliably protect at the front against projectiles (this is a "gameplay > realism" argument); if you've bought a shield, equipped it, held RMB, pointed the camera at the archer, the shield ought to work, otherwise what's the point? it's cheap and unfun if you can shoot around it easily, and there are other solid ways archers can get shots in instead; there may also be problems with hit detection in melee, I remember there could be weird moments where you'd hit someone's back yet their shield near the front magically/unfairly absorbed the hit; the same can happen with cavalry shields
Kicks
They're somewhat fine, but they should only connect if they hit your centre body (not your hand or shield), which right now can sometimes increase their range bizarrely; counter-kicking is good (it was possible in WB too); but in BL you can't jump over kicks, which was really fun and epic in WB; it's my impression that WB used something like an "isGrounded" check to see if you could be kicked; right now in BL you instead just "blob" in midair if you get kicked, and your momentum completely stops, which doesn't feel any more realistic or fun or polished
Jumping
In BL jumping is extremely lame, especially for a fully 3D game that has a lot of verticality in its maps (battlement walls, slopes, ramps, ladders, stairs, etc.), which will become even more prevalent with future custom-made maps; I liked how WB had Insurgency's approach for jumping, where it did what a jump is supposed to do (and didn't gimp it) yet you couldn't constantly do it (like CS's bunnyhopping) because of a cooldown timer, which it shared with kicking; BL's physics have an unrealistically weird floaty feel; this and other things (e.g. the ground touch after a short fall) unnecessarily make soldiers seem like 80-year-olds with arthritis rather than warriors
Edges
When you jump close to an edge, your jump is more than halved; I don't know whether this is a result of a rookie mistake relating to an "isGrounded" issue (like when beginners try to make a platformer) or a deliberate feature for the sake of supposed realism (because jumping while half off an edge might look weird), but for the sake of "gameplay > realism" (and for more WB-style gameplay) this should be reverted
Foot movement
Every single time I watch a stream of BL again after a while, the infantry movement strikes me as very poor quality, dinky and sluggish; even just for visual's sake I'd want to improve it, but for gameplay too it's like they tried making it more realistically "weighted" despite that overall it just makes characters seem strangely fat, gimped and rickety, as though moving around with molasses
Rotation
In WB's MP (not SP) the body would sharply follow the camera rotation when idle, but slow down when attacking or doing other things; in BL the rotation isn't as bad as WB's SP but a lot worse than WB's MP, since the character stupidly stands there for a second before turning (slightly); for a Warband-style mod I think the rotation should be copied directly; I can't think of anything right now that would improve upon that
Weight
WB opted for a more universal speed across all classes (possibly with very minor difference based on armour/equipment) because it was proven during tests more than 12 years ago that bigger differences of speed leads to trolling and annoying gameplay; kiting can become all too easy and people can delay rounds just by running around
Horses
BL's double-tapping feature for cavalry feels pretty good, but I'd hope something can be tweaked to make it less abusable; it might need a cooldown timer (especially the "yah" sound), so it can't be spammed constantly; I liked how horses in WB would more easily rear up when bumping into things (since it created a bigger need for awareness, so they can't just be treated like bumper cars); one of the worst aspects about BL is how cavalry can slide you around as if you're on ice; it severely messes up the gameplay and looks rubbish
Spears
Make anti-cavalry more fun to play; spears should be less floaty and more precise; make sure it's just as "easy" to rear horses as it was in WB; make it so you can rear horses while jumping (not necessarily realistic but it's fun, and it makes spears more versatile against cavalry); spears can end up a little OP in melee sometimes, so maybe something can be done about that, but the fun of anti-cavalry is the main priority; someone mentioned something about blocked stabs stunning you for a while, which might be important
Overhead thrusts
I think these are fine, and it's one of the nicer additions; it's not ideal that it can be harder to read whether it's an up or down stab in the heat of the moment, but it's not too bad either; I'm not that fussed about polearms having side swing attacks but it can be fun sometimes
Weapon stun
It's too excessive and annoying; it makes it feel like the game stopped working when it happens; maybe reduce it to just shield stunning with a held polearm overhead
Horse archers
Just don't include them (at least not for competitive); there've been discussions about how to possibly balance them going literally as far back as 2005 on these messageboards, but it's just a massive waste of time, since their entire concept is fundamentally broken for anything except a cheap SP power-fantasy where bots won't complain if you abuse it (and Captains too); maybe the Khuzait can still exist if balanced viably in other ways
Archers
I don't play this class often, but the last time I tried it in BL, it felt awful; so hopefully it can be improved to make it more fun, yet still make sure it's not easily annoying or overpowered; IG had to limit the class to 4 or 5 per team (with 50 people playing); make sure arrows don't penetrate shields, whether it's a bug or an intentional feature, it's not a good/fun feature for MP
Camera problems
Make it so releasing the tilde key makes the camera instantly go back to default view; BL's slower camera rotation adds nothing but maybe a smoother presentation for casual console gamers who argue "cinematic > gameplay"; any other functional improvements to how the camera interacts with obstacles would be good; WB's camera was pretty much ideal
Item selection
Either include an improved version of WB's gear shop (with balance and interesting variety) or vastly improve BL's system so that you're not forced to play as a pathetic peasant in baggy trousers with an elf hat choosing between either a shield or spear; provide a decent mid-build that you can always afford, and make upgrading feel satisfying yet not OP