Two Primaries (Kinetic and Energy), a Secondary (Shotgun, Sniper, Fusion Rifle, or Grenade Launcher), and a Heavy (Sword, Rocket Launcher, and bring back Machine Guns).
This way we get the best of both worlds; two primaries for general ad control, a special weapon for taking care of the slightly tougher guys, and a heavy weapon for dealing high damage to bosses.
We now get four ammo types:
White - Kinetic
Green - Energy
Blue - Special
Purple - Heavy
Suggestion for PvE:
Special Ammo will be obtainable less often than the two Primary types, but it will be available much more often than Heavy ammo. In addition, you always respawn with Secondary Ammo in PvE, like Kinetic and Energy.
All Special weapons deal bonus precision damage in PvE, and all deal bonus damage against majors and ultras. This will make them much more viable for doing DPS on bosses AND for taking out bulky centurions and the like.
Suggestion for Crucible:
In the Crucible, Players grabbing Heavy ammo get Special ammo too. Special ammo also drops for their teammates (enemies can't grab it). Killing an enemy in Crucible with Special Ammo drops Special Ammo for you. This means while it is more important to control Heavy, you can take out one enemy and then use their Special Ammo to fight back.
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This would be amazing