I am likely going to use a Mission - Mission "Debrief" structure for this game. You do missions and get back to the mission screen to proceed to the next mission. In between, you can place items in your personal storage, buy or sell weapons, ammo, and other items, or change your equipment. This can be integrated with most of the planned game modes.
Planned modes are:
Tower Defense mode. You can take the role as either defender or attacker. It may include zombies (grinding zombies with tanks?! Omg!1!one!).
Arcade mode. This is giving the player more freedom to try all available weapons, camos and vehicles, and it's going to be very similar to how the current alpha build works.
These two will definitely be in the final game. Ontop of this, I have multiple other game modes I can extent on.
Strategy Mode. I recently implemented a "death cam" that lets you scroll around the map after your death. What I could do is letting you send orders to friendly NPCs and even control them.
Campaign. The campaign itself is going to be story-driven and strategic. You will have to sneak through or shoot yourself through different levels. This is frankly where the game can possibly explode in terms of effort and time. Since I've never done a game with story before, I'll reserve the right to cut off the campaign and simplify it if I feel it's not fitting to the schedule.
Should I not be able to add a story at all, I'm going to let enemies drop notes containing some of the story in arcade or tower defense mode.
I can also directly integrate this into different game modes. Say, if you collect weapon X in the tower defense mode, you can use that weapon in the campaign. If you bought weapon X in campaign, you can use it for tower defense mode. Your inventory of that particular game save will apply to all these game modes (except arcade). So if you want to progress well-equipped (i.e. if you're playing on hardest difficulty), you can do some grinding and have some variety at the same time.
I yet have to experiment with this type of game design, but I think with the right balancing it has the potential to make the game more fun.
Combat should resemble real life combat. You take cover in order not to get hit. You use camo in order not to get seen. Your weapon behaves like a real life weapon would: it has recoil, a base bullet spread, and it also breaks the more you use it. Weapons will also be customizable using different attachments.
I'm keeping the possibility open to customize your characters as well, but I am not sure in what detail this will be implemented.