I'll jump in and post my thoughts...
In .42 I played as the cossacks a fair bit, especially in Co-Op, the rifleman (the guy with pistols...) is fast, fast helps, I generally fight with a mace, 2 pistols and bullets, using guerilla tactics rather than actual "staying power"
I find myself playing 2 player Co-Op a lot, with a musketeer, he generally dies first due to wanting to stay in the open and take pot shots.
I've played Sweden, and though they are my favourite nation, they are SLOW, for Co-Op it is far too impractical.
My suggestions:
Rename the Cossack classes, having Infantry as the ranged unit, and rifleman as the more melee based class can be confusing.
Cossacks need a tad boost in Firearms prof. Maybe melee too, I know my estimate is conservative compared to the others, but maybe 140-145?
On a personal note, I'd like to see a few more upgrades for people in Co Op mode, it's my favourite part of WFaS, and almost the only reason I come back to it (I've completed a fair few of the main story missions, and I've had my fill of M&B for now).
More customisation for Co Op would be much appreciated, having all your weapons unlocked by wave 3 makes you feel a tad redundant by the time everyone is raving about their shiny new muskets. Maybe an armour for musketeers for later waves, gloves for the Cossack Infantry. Wave 5/8 weapons for the other classes.
I know in standard MP this could be over powered, but in Co Op, I feel it would increase enjoyment and overall number of players.
Another idea, though it probably isn't possible is the idea of picking up new skills in the middle of fights, every 3 waves possibly have a choice in a rank in Iron Flesh, Power Strike, Athletics, Riding or Mounted Shooting.
On a side note, anyone have tips for dealing with mobs of spearmen? I find myself getting ganged up on by even basic spearmen, and it's hard to counter them.
Oh, and pathfinding needs to be improved, all too often men get stuck in walls or spend ages running into walls instead of going around. And how come the enemies troops in Co Op always seem so much better than the heroes men?