1. In forge you should be able to place more lights like smaller white lights. Also they need to add more vehicles like the jeep without turrets, seraphs, falcons, and also stationary cannon or artillery type guns.
2. For all game modes, when you customize your armor, each piece should have it's own special ability but with a drawback, of course to counter act it. For example, you could have extra health or shielding at the expense of speed and vice-verse.
3. The campaign mode should give you the ability to use dialog to talk to characters and possibly alter the story a little but of course without detracting from the main story.
4. Water vehicles that you can use in forge, I don't know how practical it would be but it's an idea.
5. More in-depth armor customization. I really like to customize armor but I think it could be better. In the customization screen, you should be able to select each part like you can now, but with lots more options. For the body parts it could be feet, chin guards, knee guards, waist area, chest piece, arms, wrist, gloves, helmet, and visor. You could also add detail like scratches and color each piece individually.
6. Custom Games matchmaking. 'Nuff said.
7. Some new weapons I can think of are Shock gun; it sends an electric shock but in a projectile somehow which is basically an EMP, but has better range than a plasma pistol and less recharge time. Bring incendiary grenades back just for the lulz.
That's all i can think of for now, if anyone has any more suggestions, feel free to post them on my topic