Few things to consider maybe adding, a better more expensive version of buffout.
A "Enemies left" counter per wave
Kill x of y could also reward small perma upgrades like +10% damage vs them
Kill x with z weapon, similar, + % damage, or recoil reduction or discount at the vendor
A small one but not sure how difficult it would be to implement, a in-round pause with access to the options, audio, controls ect, I'm always forgetting a button
Gallery mode? I'm not sure its implemented and I'm missing it, as the data counts are awesome, playing the animations would be cool too after unlocking them.
At the moment the cash value is "vulnerable" to cheat engine, something to note if something like that bothers you - tested to try out some of the more expensive vendor items
Keep up the great work!