As someone who has played KZ2 for over 500 hours here are my suggestions:
Technical:
Lag. Lag is a big issue in KZ2. Ranging from a simple annoyance to a game damaging experience. Please do whatever you can to reduce the amount of lag present in the game. And please use dedicated servers for KZ3 next time.
Gameplay:
The Assault Class. Extra armour, boost ability, rocket launcher. Just horribly unbalanced class. Not only that but you can swap boost with any character eventually and there's no limit to how many rocket wielding assault classes you can have on one team. Have you ever played a game where everyone has the boost ability no matter what class or that half the other team just spams rockets everywhere? It happens a lot and ruins the game. I'd say it's what has put the most people off the online experience so it's a big problem.
Despite that though I do recognise that the Assault class does have two distinct uses. It is used to break through turret defenses and can stop S&R ending in stalemate on larger maps due to their boost ability.
My suggestion would be to split these two roles up among two separate classes:
1. The Demolitions Class - EMP grenades with a larger blast radius than normal grenades which do extra damage to turrets and minimal damage to humans. EMP launcher to take down Air Drones which also does minimal damage to human players.
2. Assassin Class - 20% less health - Boost as primary ability so you can't give it to other classes and the ability to use knives in melee.
These are just off the top of my head but I think getting rid of the Assault Class for something like this would make the online experience much more balanced and enjoyable.
I hope you like my suggestions and I want to finish off by saying that KZ2, despite it's issues, is my favorite online shooter ever and I'm sure I'll be increasing my playtime beyond the 600 hours mark in the future
Cheers.