Oxygen Tanks
Oxygen should be a consumable resource than a fixed match timer. Although 25 minutes might be enough for some, the fixed timer limits users from implementing their own unique play styles. I think that a good change to them would be to implement an oxygen tank system, where players would spawn with a default 25 minute oxygen tank, but could purchase oxygen tanks with higher time limits. I would also want to suggest maybe being able to "steal" oxygen tanks from other dead marauders (not bots), however I feel like it could be a little too powerful, so maybe just being able to buy them from the store could be a balanced addition.
Maps/Helmet
This might be pushing it, but I think that a high tier, expensive helmet that has a bunch of features could be cool. Maybe it has a built in map, ammo counter, or NV attached? It fits the lore and general dieselpunk vibe to it. Speaking of maps, although Marauders is sticking to a "tactical realism" kind of genre of FPS, I believe that it would be beneficial for there to be some kind of map, whether it's on a wall in the playfield itself, or maybe a purchasable map that you have to equip to your inventory.
Insurance
I believe that there should be some form of insurance system in the game. The insurance cost could be progressive; meaning the higher tier loot that you want to bring into a match, the more expensive said insurance would be. Insurance could be for both items and potentially ships (ships make more sense for there to be insurance for, due to their absorbantly high cost).
Sights
Sights should be universal on every gun. A couple guns have them, but they don't magnify that much and are generally a big waste of money. If I could change them, I'd probably add the ability for sights to be universal and not to be stuck to one gun.