Optimizations - Literally every game better be making use of this. Do I need to say why?
An actual active dev team that LISTENS TO THE COMMUNITY - I don't know what's going on for KF2 for the last few years, but the devs seem to be listening to nobody but themselves. Nobody likes having to pay $10 for a single DLC, nor were people stoked starting Summer Sideshow 2018 for what was added. A good example can be an Alpha stage where the devs and player have direct contact and can share and give opinions to at least get an idea of what people want in KiIlling Floor 3.
Increased body health (and resistance to bodyshots) for Zeds - KF2 already has a massive problem with vanilla Zeds where a chaos perk comp can just cheese body damage even groups of Fleshpounds to death, and how Scrakes can easily be bodyshotted by how easy it is to afflict them It should take a good while for a body damage comp to jump a single FP. FPs and Scrakes should be able to resist enough bodyshots to punish the team heavily assuming bad play, while trash and medium Zeds resist multiple bodyshots.
Increased Zed damage (and range) - Assuming medic will have buffs in KF3, Nothing can hit hard enough to outpace the Medic's healing and buffs as it current stands in KF2. You can literally stand in multiple sirens and still be relatively fine on HoE. No Zed (save for Sirens) should be able to one-shot a player with any attack, but a combo like say a raged Fleshpound attack plus a shot that directly hits a player by a Husk should down a player, or at very minimum leave it on low HP. Sirens should devastate any unprepared team by leaving them all on very low HP, Husks should have increased accuracy and shooting cooldown.
Increased Zed speed - As of currently, a Gunslinger with full medic buffs can outpace a Fleshpound, this should not be the case and will promote more careful gameplay rather than mindlessly running around like headless chickens and being able to survive.
Focus on precision-based gameplay - KF1 might of been slow, but in that game you had to play for headshots on 6p HoE to succeed. In KF2 you can bodydamage your way to success. Why should it be easier and even promoted that playing like a complete bafoon gets me easier success than not?
Giving the Zeds subtle, but very game changing abilities on higher difficulties. - The "Nightcore Zeds" are a great example of giving abilities to a Zed to make them far more dangerous to play mindlessly around. Scrakes there will inflict a slow on a player (while also having more reach and damage) on a hit. More examples that can be used is the VS Mode Siren "Vortex" ability, sped-up Nightcore Fleshpound and Husk animation, or the Crawlers more focused running on walls and ceilings to let the other Zeds charge into players without clogging the way up.
Increased amount of Zeds alive and allowed to spawn at once - As of now, 32mm (with it decreasing all the way to 12mm) is too easy to play with, the only spawns worth a concern for a team as it stands is a triple FP spawn.
No Zed downscaling of any form per dead team player - This should demote any selfish playstyles and force actual cooperation. No more Zerk/Medic 20 minute kites per wave.
Animations Killing Floor 2 had - One of KF2's biggest appeals is the gunplay with mocap animations and 240 FPS.
No EDARs/Spawnrages - Ask anyone actual vocal about the highest difficulty and I can guarentee these two factors are on the top list of hated mechanics. No mechanic should rely on pure RNG to increase difficulty such as spawnrages, nor should enemies never die to headshots (EDARs)
Less is more - Think about all the newly added content from 2018 and beyond, how many of these weapons are actually seeing usage? How many are actually able to add themselves to the meta? This shares a similar problem to Payday 2 where there's so much variety, but only 10% of all the weapons actually see usage on Death Sentence (highest vanilla level) play. Or how about the Zed variants which are hit or complete miss? How many people actually like gas crawlers, QPs, or EDARs? Or how about the cosmetics which love to eat up GBs of space. Does the upgrade system serve any real purpose apart from getting certain weapons unnerfed?
Make skills have a real purpose - Right side medic (which is trolling btw), Vampire, Resistance, Fortitude, AP Shot, you get it at this point. How many people actually use these skills?
There are far more things I can list with more time but I think I listed my thoughts and opinions for what KF3 needs.
An actual active dev team that LISTENS TO THE COMMUNITY - I don't know what's going on for KF2 for the last few years, but the devs seem to be listening to nobody but themselves. Nobody likes having to pay $10 for a single DLC, nor were people stoked starting Summer Sideshow 2018 for what was added. A good example can be an Alpha stage where the devs and player have direct contact and can share and give opinions to at least get an idea of what people want in KiIlling Floor 3.
Increased body health (and resistance to bodyshots) for Zeds - KF2 already has a massive problem with vanilla Zeds where a chaos perk comp can just cheese body damage even groups of Fleshpounds to death, and how Scrakes can easily be bodyshotted by how easy it is to afflict them It should take a good while for a body damage comp to jump a single FP. FPs and Scrakes should be able to resist enough bodyshots to punish the team heavily assuming bad play, while trash and medium Zeds resist multiple bodyshots.
Increased Zed damage (and range) - Assuming medic will have buffs in KF3, Nothing can hit hard enough to outpace the Medic's healing and buffs as it current stands in KF2. You can literally stand in multiple sirens and still be relatively fine on HoE. No Zed (save for Sirens) should be able to one-shot a player with any attack, but a combo like say a raged Fleshpound attack plus a shot that directly hits a player by a Husk should down a player, or at very minimum leave it on low HP. Sirens should devastate any unprepared team by leaving them all on very low HP, Husks should have increased accuracy and shooting cooldown.
Increased Zed speed - As of currently, a Gunslinger with full medic buffs can outpace a Fleshpound, this should not be the case and will promote more careful gameplay rather than mindlessly running around like headless chickens and being able to survive.
Focus on precision-based gameplay - KF1 might of been slow, but in that game you had to play for headshots on 6p HoE to succeed. In KF2 you can bodydamage your way to success. Why should it be easier and even promoted that playing like a complete bafoon gets me easier success than not?
If Chaos perks have to exist, then make it a state to where it's objectively inferior to precision. Considering the state of current KF2, chaos comps should never be able to be able to beat the highest difficulty without significantly more struggle than precision.
Giving the Zeds subtle, but very game changing abilities on higher difficulties. - The "Nightcore Zeds" are a great example of giving abilities to a Zed to make them far more dangerous to play mindlessly around. Scrakes there will inflict a slow on a player (while also having more reach and damage) on a hit. More examples that can be used is the VS Mode Siren "Vortex" ability, sped-up Nightcore Fleshpound and Husk animation, or the Crawlers more focused running on walls and ceilings to let the other Zeds charge into players without clogging the way up.
Increased amount of Zeds alive and allowed to spawn at once - As of now, 32mm (with it decreasing all the way to 12mm) is too easy to play with, the only spawns worth a concern for a team as it stands is a triple FP spawn.
No Zed downscaling of any form per dead team player - This should demote any selfish playstyles and force actual cooperation. No more Zerk/Medic 20 minute kites per wave.
Animations Killing Floor 2 had - One of KF2's biggest appeals is the gunplay with mocap animations and 240 FPS.
No EDARs/Spawnrages - Ask anyone actual vocal about the highest difficulty and I can guarentee these two factors are on the top list of hated mechanics. No mechanic should rely on pure RNG to increase difficulty such as spawnrages, nor should enemies never die to headshots (EDARs)
Less is more - Think about all the newly added content from 2018 and beyond, how many of these weapons are actually seeing usage? How many are actually able to add themselves to the meta? This shares a similar problem to Payday 2 where there's so much variety, but only 10% of all the weapons actually see usage on Death Sentence (highest vanilla level) play. Or how about the Zed variants which are hit or complete miss? How many people actually like gas crawlers, QPs, or EDARs? Or how about the cosmetics which love to eat up GBs of space. Does the upgrade system serve any real purpose apart from getting certain weapons unnerfed?
Make skills have a real purpose - Right side medic (which is trolling btw), Vampire, Resistance, Fortitude, AP Shot, you get it at this point. How many people actually use these skills?
There are far more things I can list with more time but I think I listed my thoughts and opinions for what KF3 needs.