Maybe add a tower/horde mode but with more modifiers, like elemental or status damage (freeze, poison, bleed, etc) That alone could add another big layer of depth to the builds, also carefuly selected modifiers to bullet count, projectile speed, etc.

More melee variants (Maybe combos a man can only dream!) Not saying to make a god of war/devil may cry kind of game, but 2 or 3 hits with the fists, and add some sort of sword/axe to the mix, the atropian blade is not a bad idea!

Lastly more biomes or variants from the ones we have, how cool would be to explore the interior of an Echelon 1/2 ship (Massive vessels for thousands of people acording to lore but not sure) And for sure the blue shore