Okay, so I like how there is a leveling system. Maybe each level has something you can unlock, and some items in stores can be for certain levels. So an example could be the one bike in missions. You have to be Level 6 (Warrior) to ride it. That could apply to items you can unlock when you level up! Also, when you level up a screen can pop up, and say something like:
You have leveled up to level 6!
Congratulations! You have unlocked the following items:
-Christmas Rifle
-Smoke Grenades
-Alien Gun
-Poisonous Knife
What You Will Unlock At Level 7:
-Grenade Launcher
-Stun Grenades
-Metallic Rifle
Also, you know how there is a scoreboard for people who have the most kills a day and all that? Maybe, if you click your level on your profile, it shows you ranked compared to everyone else, using the level you are. So, those are just so ideas I thought of, I'd love to hear some feedback! Thanks.