I managed to come up with a few more things to add.
1. There needs to be display resolution options. I had to zoom out on Dinogen Arena because my 1366 × 768 monitor couldn't fit it all within the browser.
2. More enemy variety would spice up the game-play a bit. Zombies, for example, can come in fast but frail versions or slow but tough variants that vomit a stream of bile up close or globs from a distance. You can also have drones that fire bullets or self-destruct with explosive force or lingering flames.
3. Powered armor could be unlockable cosmetics alongside normal ballistic protection.
4. Mini-mechs and spider-tanks can be vehicles distinct from the normal armored variants seen in most standard shooters.
5. Try looking into more varied gun cartridges beyond the usual encountered in most shooter games. Military Wiki has a decent list on cartridges between 4mm-13mm.
6. Similarly, you can add more weapon variety. I'd like to see lever-action carbines and anti-tank rifles make an appearance. Forgotten Weapons has plenty of unconventional and unknown examples you can use for inspiration.
I hope this list may be of good use to you.
Belyaevs-Fox
Multiple shotgun calibers would help diversify the gun-play in meaningful ways. Most games only cover 12 gauge while neglecting .410 bore and the range of 28-8 gauge. You can also add different ammo types like hollow-point bullets and shotgun slugs.
Shields could be used in single player as a defense against foes on one side while leaving the other side exposed, leading to some interesting tactical decisions.
Different grenades can be used as a supplement to the guns, like conventional explosives, EMP, flash-bangs, and incendiary.
Melee strikes with guns would be useful to conserve ammo or as a last resort attack. You can even add bayonets to increase strike damage and more melee weapons like tomahawks, kukris, and entrenching spades with different properties.
And lastly, game-pad support would be much appreciated, since I have a PS4 controller while many others would use Xbox or Gamecube/Switch Pro controllers.