I log here only because I like this mod a lot. It feels like professional game from 90's and I feel I want to play it more.
I am long time lurker here but now I have reason to write something, and it is long so I will start from my humble opinion on what was made already. I love how different levels got different gameplay mechanics: simple fps exploration, bike section, scary part in subway caverns, functional towns with NPC characters and that radiation in The City map.
My opinion on different levels:
0. Tutorial - good level, nothing to change
1. Outskirts - great level, nothing to change
2. The Boneyard - great level, nothing to change, I like some small changes since demo version.
3. The Wall - small location, but still functional. Could have more details, like extra rooms/floors ,more posters and furniture.
One of traders on street could sell armor scraps and bigger armor for higher price,
ammo backpack for even higher price (it should be the most expansive item in town that You could buy only if You gather all scraps on two first levels)
or weapon upgrades like extended mag for pistol or shotgun.
People on streets could talk that at least they have electricity and clean water for free.
I would love to see some working machinery, pumps and water tanks in the dam, workers in orange suits and yellow hats,
maybe fields or farmlands on additional street.
Maybe extra room where town guards would take all Your collected scrap and exchange it for money.
4. Highway Holocaust - great level, not that much to change. Most important would be to fix damage per bump into walls on bike, lower it.
Some smaller not necessary changes would be nice - add few more bushes and grass here and there,
made some rocks not that flat (in comparison to Duke Nukem Forever build engine mod, level on bike).
Just when You start level there is road that turn right and ends near ruined building - why not add there collapsed tunnel.
As for police station exploration that part was perfect and there is nothing to change.
5. The Sub-Caverns - great moody level, nothing to change.
6. Temple of the Dollar - nothing to change, it is perfect.
7. Terminus - bit too small, still functional. I would love to see some extra rooms like toilets with working mirrors.
Please make this location more vertical with two more floors - for different metro lines. like You start on lowest part, on mid there are shops/bar and exit is on top part.
Add some dead ends corridors with locked doors and bandits guarding them - maybe those are different gangs headwaters.
Add some rooms where people cultivate mushrooms or keep rats for food.
It would be cool if Violet have some side quest for player to find some stuff and improve her bar.
8. The City - another perfect level. Nothing extreme important to change.
Some smaller changes could be made - I would love to see more open spaces with ruined buildings to explore or more opportunity to use rifle with scope
I would love to see some wrecked military jeeps and transporters near radio tower,
maybe some destroyed military camp with extra ammo and extra floor in radio tower with mutant nest,
You know in The Boneyard and Sub-Caverns there were those rooms adapted for "kitchen" by mutants.
9. Damage Control - this is good map for me.
It could be cool to add decontamination corridor just after those code secured door and small security checkpoint room.
Add isolation room and room with few beds to medical wings, those rooms could be half collapsed to show player that bunker is in bad shape.
Near final part there could be giant room with server farms, full of big blocks computers that use giant tapes for storage media, also it could be half collapsed.
I would love to see room, even if it was not accessible/behind the window, with giant spinning hard disk plate and mechanical hand - human size

.
Weapons are great too, they feel right. It would be cool if they were bit weaker (shotgun and pistol could have smaller magazines) and could be upgraded to current state in first town.
Ideas for weapons:
M16 - Hidden m16 with extra even more hidden upgrade for grande launcher.
Cross bow - silliest weapon that can kill without alerting other monsters, scare ammo and very small amount of ammo to hold.
Extra upgrade with explosive bolts or chemical bolts that can paralyze group of mutants.
Nail gun - tool used for building adapted to be weapon, fire nails, when upgraded could fire even faster or have extended mag.
Pneumatic hammer - already in game, fix infinite ammo to actually go down and regenerate very slow to not be that much over powered.
Maybe add extra push for dead enemies.
Molotov cocktails - grenades that explode on impact and set enemies on fire.
Solar lamp - since it is hold in one hand it would be cool to have also male attack with crowbar.
Ideas for more enemy types:
some freak of nature mutant that can teleport
female bandits
different skin bandits
heavy armored bandits
bandit boss
biker gang that wear leather jackets with spikes, got fat members
tribal gang, very acrobatic, aggressive, can jump high, throw knives, run fast, some use normal guns, some rush on player
military gang, they can throw grenades if they do not see You, seek for cover, crouch when firing or lie on ground like pig cops from duke
monks-cultists gang, bandits that wear habits and hoods, like cultist from first Blood, they could telepathically control mutant cannibals, also be very defensive
rather then attack player on sight, stay in one place, continue attacking, they would attack and try to run away
mutant dogs infected with that green explosive thing, when killed they will explode
suicide mutant cannibals with explosive pipe bombs that run in player direction
mutated fishes in underwater parts
female assassins gang, flexible, can teleport, just like half-life female assassins