My first suggestion is to give unlimited ammo for whatever gun player could wield in this game. This will allow player to express themselves freely without worrying about ammo.
Speaking of gun, my second suggestion is to give various guns for players. At least these 3 guns:
1. Default gun, shoot rate = medium, knockback = medium, damage = medium.
2. Shotgun, shoot rate = low, knockback = high, damage = medium to high.
3. Ingram, shoot rate = high, knockback = low, damage = low.
As for enemies, currently I have two types in my head:
1. Chaser; this enemy moves horizontally but will always face player and try to touch player to damage player.
2. Upshoot; this enemy moves horizontally while shooting upwards periodically. It doesn't care where player is meaning it won't chase player.