Hi. I am going to propose several ideas to improve some mechanics and functions of the game that I think have a lot of untapped potential and are easy to incorporate from my point of view.
1. Personalization of war vehicles; Being able to incorporate turrets, saws, drop bombs ... etc
2. More customization for weapons; Being able to add different chargers, grips, buttstock... Add option to put knives on certain weapons, first person sighting sight, studded baseball bats with nails and wires (Lucille?). Maybe add key chains and collectibles for weapons and backpacks?
3. More dynamic and varied missions; Adding perhaps Human attacks to your base or vice versa, hostage rescue in fortified bases...
4. A new cooperative game mode (Inside Daybreak maybe?) Where you have to enter or exit a government or secret facility with a countdown. Full of hallways, rooms, equipment to help escape or enter (and zombies obviously). This opens up many more possibilities, such as opening doors with puzzles, while colleagues cover their backs. If you can reward players with points to use in your community or only unlockables in this way to boost online play. Another idea is to incorporate the same idea but with two teams in PvP, Attackers and Defenders, but I understand that this part is much more complicated to carry out.
5. I would bet on some interface changes such as being able to see your weapons and equipment in their real form without having to equip them, if you add the function of completely customizing them as I mentioned before it would be a good way to see everything clearly.
6. Procedural maps and even more random elements to make it more replayable (bases in random positions?). One option is to incorporate a community map creation system, where they would be delighted and you would have an endless amount of free content for you.