First of all thanks for the nice game playtest. I played around 4.5 hours and lvl 16 atm. I would say the fundamental gameplay is strong and it feels like the game has implemented some things from the successful 4-person co-op shooter games. But from my perspective, the game needs more unique features based on this ground.
First of all, the game has a generic flow of: endure the waves of enemies and get something for boss, go for boss, escape. This doesn't change although the contents are a bit different while doing this kind of events, such as running the TNT factory or killing certain enemy numbers. The main problem with the same flow of the gameplay is that it feels like that every mission looks the same. Moreover, lack of boss roster makes it more evident. I like the random side quests, but as we have to go through this generic flow after all, the game feels quite repetitive. In this regard, you might want to benchmark or get inspiration from other games, such as DRG, to expand the selection of mission and the game flow.
Secondly, I find the weapon upgrade to be quite generic as well. Especially, with the ability(forgot the name, that you can equip max. 4 slots with certain abilities) that gives you 50% faster reload when your mag is empty, already makes the reload speed perk of the weapon nearly useless for what it gives, and for dps it's already determined to increase fire rate/damage anyways, so it feels like it's not very fun to choose something. I recently played a game called mycopunk, which is also a 4 person shooter, and it had a very innovative system to do some kind of hexagonal tetris where you can build your own weapon to create hundreds of variety of your preference. For now, there are not much to farm in the game, and at my level, I found that I don't really need more ingame currencies including soul and gold anymore after buying out all 4 shop purchasable skins and upgrades so far. In this regard, it would be great if there is a random upgrade