its fairly obvious that each of the crystals seen all levels of the game represent a different damage type. red: normal, yellow: piercing, purple: shadow, green: elemental. the only one that has no damage type is blue. i figure the fifth damage type could be something that deals light damage (as an opposite of shadow). this will also give OOO a chance to revamp sleep vials (since no one likes them anyway) change the sleep vials to enemy confusion (since the enemy will 'see the light') causing enemies affected to become a temporary ally.
also, the fourth weapon class. two thoughts went through my head on this one: ranged explosives, or a healing ability. since we already have bombs in the game, i guess ranged explosives can go by the wayside. the bigger problem is being able to keep people alive while fighting. i cannot provide a description of what healing items should look like, but i figure it would work on a charge like bombs do. while some reading this may feel that capsules would become obsolete or an item of non-use. i would have to disagree. maintaining health can be very hard, even with the right armor for the foe, so this provides a buffer of sorts if you run out of capsules. the big issue here though is how this would affect resurrecting allies. i will leave that to you to discuss.
Sure, the Purple Dark Matter crystal can be tied to Shadow damage, but here's the description for the yellow one.
"A fabled mineral of a radiant, golden color said to contain the faith, wisdom, and zeal of the life surrounding it. Small pieces of Luminite called Light Shards are used in crafting." Yeah, that sounds like piercing. All toothpick users do is slash around anyway. ;D
And then here's the green one.
"A fabled mineral of a vibrant green color said to contain the essence of all natural life. Small pieces of Valestone called Green Shards are used in crafting. " All natural life? Yeah, Gremlins, their war machines, and constructs are pretty natural.
Ranged explosives undermine bombs. Healing will NEVER be happening (I think the devs stated it once). Light Damage would screw up the cycle, either causing a need for it to be revamped, or pinning TWO weaknesses against the Undead and Fiend families. Besides, we have elemental damage.