Attack and block animations should not depend on the server, and instead trigger immediately when the player presses a button. It helps the player to control over the character with more comfort while the process of adapting to a ping would still be the same.
Now I am gonna present some debatable suggestions since I believe the combat should not be a copy cat of M&B's combat:
1. I am fine with the current spinning speed. You punish the player for missing, keep that. No need people spinning all the time. However, perhaps allow the player spin fast if blocking?
2. When blocking an attack, let the attacker be stunned a little longer but allow him to move to at least dodge attacks.
3. Improve chambering, I haven't tested as I don't have good testing grounds but I believe a "ping-pong" situation where players chamber each other should be possible. That can be accomplished by allowing more chamber control: You decide if you want to release to attack, or stop the "ping pong" to go for a block. Congrats, we just created a rock-paper-scissors game, except it's attack-block-chamber.
4. Related to 2: as a blocker, perhaps gain small damage bonus for a successful block? Special animation for that?
5. a. Add a stock strike to stun a player who's turtling (blocking all the time and not moving). Very short range but degree angel would be about 170.
5. b. If not stock strike, than a kick. Range to whatever you think might be right here, I'd still recommend short, and obviously angel would be the aim.
5. c. Add both, but stock strike will be available only for short barrels and pistols (as elbow strikes). This kinda gets a mechanic that is similar to the grappling mechanic in fighting games. Recovery rate after such strike should depend on the weapon (in other words, long recovery for muskets if you include it for it)
6. Not sure if it exists already, but bonus damage for offensive gameplay. More damage when charging into enemy.
7. More action-canceling moments to create confusion. Maybe this already exists, but it's hard for me to notice (or "feel") due ping.
Mechanics for crouching in melee combat: (easier to mix crouch into combat if crouch is not toggled)
1. To use it to dodge upper stabs, allow crouching while moving and allow it to be "canceled" in middle of crouching so you can immediately stand up if you decide to do so in the middle of that action.
2. If kicking added, allow to do it while crouched.
The reason I think crouching should be used in combat is because it would work similarly to double tapping a button in other fighting games to dodge, except in this case you don't double tap and do it yourself more actively (which also means the attacked might also still hit you). Double tapping would suck, but this mechanic would add more variety to combat.
I believe most of the things I added above would work well if you also make it possible for the player to cancel or combine (mix) their actions at will, unless a certain time passes. It makes the fighting more dynamic.
P.S Sorry for the grammar mistakes, each time I read my own post I find more and more mistakes. If something's not clear, I'd happily rephrase it.