I really enjoyed playing Survival mode so far, but it kinda felt lackluster in the long run and less rewarding at the end. I have some suggestions if you're interest adding them in the future update.
-Highscore and combo multiplier system.
-Display your highest waves/score and win/loss on the Survival menu.
-Shows all weapon/perk locations when selecting or loading the map.
-AI has a random chance to spawn an upgraded variant above wave 10.
-Money cap setting on how much money you can carry.
-NPC movement speed setting.
-Additional NPC spawn multiplier.
-Headshot deals extra cash by 1.25x
-Last enemy indicator.
-Knife auto pickup.
-Completing wave 20 or reach certain score unlocks achievements and bonus options:
*Kamikaze/unpinned: When the NPC dies, it drops a live grenade.
*Randomizer: All NPCs (zombies/soldiers) and weapon stations are randomized and give a random weapon for every 10-20 kills. Chances you get an upgrade variant to be tolerable on a higher wave.
*Limit Breaker: Increase effectiveness by 25% for weapons and perks (prices increase 1000 for each purchase).
*666 - Enemy spawn are doubled
Changes:
-Venom and Tesla gun can not be obtained from the random box in the early game until wave 10 to match the difficulty scale.
-Buff the enemy shotgun to 50 damage to pose a major threat.
-All Pistols, reduce the upgrade bench to 500, this encourages you to utilize more efficiency in the early games.
-Increase the price for the pump shotgun to 600.
Give classes negative traits that respectfully fits for the perks to compensate:
*Engineer - Power up has a shorter duration.
*Medic - Armor cut in half.
*Soldier - Stamina drains faster
*Lieutenant - 10% reduction for RoF
*Covert Ops - slightly faster mobility but at a cost of your health to 80.

i like to some some where your a german soldier taking on the allies .

I'll take a not of some of these, thanks.

Honestly, I love survival but the one thing that's pissing me off is the deeper you get into the waves, the same or even worse the weapons get via the randomizer.
it's a slim chance in hell you get a rocketlauncher, 500 ammo machine gun, flame thrower or tesla gun, and these are legitimately the only weapons that give you a chance past wave 10.
Getting very frustraed when I wanna switch out a mid machine gun, even after wave 7 or so, and get hit with a hand gun or single shot shotgun 9 times out of 10 - against a group of 20-30 enemies.
I also like your idea of keeping track of wave/score records. I got to Wave 24 of the river post with over 1500 kills (me 1000+ plus, allies 500+) and my buddy doesn't believe me lol. Oh well

One thing I did find that was fuggin awesome, isvwhen you upgrade the tesla gun (the 2 times I ever got it lol), it turns into a flame thrower too, sort of. lighting enemies on fire. So hard to get a tesla tho + the 1000 pts needed to upgrade. that's an elite weapon. and even so, it'd still be a challenge after wave 15. but a fun, realistic challenge.
I also like the headshot 1.5x pts and didn't even know there was a cap on pts/money?
highest I got was 4400+ during my river post PR.
is it 5000 or? I personally didn't find I needed more than that, but that's just me.

Additional suggestions:
Changes:
-Venom and Tesla gun can not be obtained from the random box in the early game until wave 10 to match the difficulty scale.
-Random box prices increase by 25% for each purchase, and price resets after the next wave.
-Buff the enemy shotgun to 50 damage to pose a major threat.
-All Pistols, reduce the upgrade bench to 500, this encourages you to utilize more efficiency in the early games.
-Increase the price for the pump shotgun to 600.
-Increase the upgrade price to 2000 for the Venom and Tesla gun due to their insane powerhouse.
-Give classes negative traits that respectfully fits for the perks to compensate

BTW Tesla, flamer and Venom are not obtainable from the box until wave 5 in the recent update

Realism difficulty for survival would be great.

Also, if I can be shot while reloading, enemies should be able to be shot while reloading as well... Far too often, an enemy can take like 3-4 head shots and not even be affected because his mid reload.