Well, we just had a discussion about how impactful such changes could be in a franchise like Killing Floor, but since it was more focused on increasing movement tech rather than making the maps and their environment more interactable... I guess it warrants a few extra tidbits of information. Even if the general concerns (perk roles, weapons functionality, zeds power...) are still overall applicable.
-Broken Doors : well, unlike the first Killing Floor, KF2 really didn't invite us to use doors as a tactical advantage no longer. Considering how sluggish you were in KF1 (as well as zeds), it was pretty natural to deliberately choose to box-in and cover a small area with all your firepower... Made even easier thanks to doors that would magically respawn after each waves, but also pipe bombs, grenades and whatnot. In KF2 though... Unless I was trying to be a little cheeky bastard by placing booby traps as a Demo, they were pretty much useless. Even with a diligent Support Specialist on your team to repair any broken doors, you were often better kiting around the map rather than camping a single zone... Especially with zeds being a bit meaner this time around. If anything : sealed doors could spell your demise instead ! As it effectively removed a possible escape path for you and your mates to take. Unless KF3 goes back to that slower pace (and I don't know if that would be a great idea), I don't think we should bother with doors. That would probably barely change anything, except for a select few clutch seconds you might win by closing the door on a zed's face.
-Breakable Walls/Floor Gaps : now we're into much more interesting and uncharted territory. I could totally see it being both a blessing and a curse. A blessing as you can take zeds by surprise and even open up a new path for you to flee out. But also open up a new gate for zeds to swarm you quicker. The fact that it's preset is a little reassuring, as you couldn't just tear down a whole entire building (especially by mistake). And I'm personally all for maps that "evolve" as you're playing them. I do fear however that it could bring up some griefing potential, with lone teammates taking the initiative to tear down a wall at an inopportune moment. The fact that crawlers could also open up such walls is also a little worrying, considering they're among the most plentiful of zeds. I believe it should be reserved for PLAYERS ONLY, with the added balancing being that once a new path is open... it is open to everyone. Fiends and foes alike. I included your Floor Gaps idea to the mix, as I have similar thoughts regarding them.
-Crawler Tunnels : let's cast aside the mere thought of it being far too difficult to implement in a way to feels natural and instead focus on what it could bring should the ability be added to the game. I think it's better than giving Crawlers the ability to tear down walls like players does. And I also believe it's cool to have a zed with a more supportive role towards its brethren (I believe the only zeds coming close to that idea right now are the Rioters and Sirens). It would also feel a litte less jarring than zeds teleporting to your position... And I could see it as a nice "bonus objective" to shut down tunnels by lobbing grenades or fire into them. But I also think it would look a little comical to see a massive Bloat or Fleshpound emerging from a tiny hole built for spidermen... Maybe these could be limited to trash zeds only? It would also avoid suddenly having a small army emerging straight from under your feet. Similarly to the Breakable Walls idea though... I think it should be limited to Zeds only. Not the players themselves.
-Support Specialist : You haven't defined what you mean by "vent shafts", but I would guess it applies to smaller zeds spawning from sewers or well...vents. I'm not exactly against it, anything that makes the Welder a little more helpful is welcome. But I honestly think it's gonna be the same deal as doors : most people wouldn't even bother, and instead will try to dispatch the zeds as soon as they take a little peek. If it forced crawlers and such to take OTHER PATHS however... You might have a little strategy here, although unreliable.
-Interactable cars : Again, I fear that it might be way too cumbersome to actually add into the game. Since it's a shooter and not a racing game, I can't see them taking extra care into making the cars physics and driving optimal. Because if they did, well... might as well make a racing game. And if it's too annoying to use, people might just not bother using the mechanic. I also feel that it could end up getting you killed more than it would save your skin. And last (but certainly not least) : think about map making. Usually, props are not just there to decorate an otherwise fairly empty map. They often act as some sort of obstacles or barriers... derelict vehicles being part of the equation. By allowing them to be used, you're effectively reshaping a significant part of the map layout... And unless those become super massive (which doesn't seem to mesh well with KF's gameplay), you'd effectively make something that wouldn't amount to much in the grand scheme of things. I'd be okay to turn them into booby traps as a Demo though... similar to how doors can be used. I just miss being able to place traps man... C4 just isn't the same as pipebombs !
-Crawlers in general : Don't get me wrong, I'm all for novelty in sequels (and gaming in general). And just like KF2 added some flair to nearly all existing zeds, I think KF3 should try to do the same. And also, if we can get new biomes, that's the icing on the cake ! I'm tired of labs and military bases to be frank. But having maps based almost entirely on crawlers? Giving them set-spawn points that would infinitely spawn them until you dispatch their nests? Giving them the ability to fly? All of that sounds like a lot... Every zed in Killing Floor so far sort of fits a role. The Husk is there to keep your team in check at longer ranges. Bloats are bullet sponges. Sirens are here to nullify explosives and gnaw straight at your health. The Fleshpound and Scrakes are the tanks and so on. Having crawler-focused maps would greatly weaken certain perks and make some others downright invincible. Going after nests could become either trivial or downright impossible. As for them flying... I guess it could spark some new life into the game's slowly aging mechanics. But it would require such a new perception of how maps are supposed to work. And while you don't really need a lot of vertical mobility to aim up and shoot... I can already picture zerks having an awful time dealing with those flying threats. I do concur however that their swarming nature could make them the next game's Clot... who honestly aren't really working as a "cannon-fodder, but dangerous in packs" enemy.
-Nest Guards : OK... I assume you got inspired by antlions from the Half-Life universe? Good choice ! But I can already see a few issues with such an enemy. First and foremost : making it more difficult for precision perks, when those are already meant to be "high-skill, high reward" type of classes is a little dumb. You should reward players for careful aiming their shots, not punish them ! It's already hard to convince people to play Sharpie or Gunslinger in KF2 when you can do the same and even more with a Demo or Firebug... I feel it would dilute their interest even more. Similarly : making the body the main weakpoint rather than the head is something that doesn't really sit well with players, as EDARs proved. I'm personally NOT against games keeping you on your toes and making you relearn some strategies when new challenges appear. But here, it almost feels as if you're asking players to be DUMBER in order to prevail. It's ok for the Firebug, as aiming for the backpack is a little trickier at times (it's behind its back after all...) but rewards you with a big explosion. It is not when headshots are suddenly invalidated and going rambo is the best way to go. And yet again : I believe Bloats, Scrakes and Fleshpounds are already great exemples of tanks for the Zed Horde. You KNOW they are top-priority targets that can quickly demolish your entire team if you're not careful. But focusing only on them can allow other zeds to move forward, potentially reaching you and trap you... The Nest Guard sounds like more of the same, with just a little annoying tidbit borrowed from the EDARs. Maybe allow FPs and Scrakes to block incoming damage a bit more frequently, and I feel we'll have achieved nearly the same as your new zed idea... Sorry !