Raids plays a big into making players group for a common achievement. I won't touch into epic, as those who have an epic version can still be attractive for some items. This proposition is only for heroic versions.

The raids would give a new loot called: a Stone, specific to the raid. If an epic version of the raid exists, this item would drop in addition to the usual stuff, following the same drop rate as heroic difficulties.

This would be an ingredient, that you could combine with other stones to produce some unique consumables. Because they are consumables, you would still have a reason to do the raid. It's also important that they give unique effects, to not suffer from concurrence (they could be imitated by Store Potions though), and that those effects would not affect power creep (i.e. no +X attributes, spell power, etc.).

Plane of the Night: Draconic Stone
Zawabi's Revenge: Demonic Stone
Titan Awakes: Mechanic Stone
Ascension Chamber: Undeath Stone
Reaver's Fate: Gigantic Stone
Hound of Xoriat: Madness Stone
Vision of Destruction: Devilish Stone

The drop rate would be:
Normal: 30% of getting one
Hard: 60% of getting one
Elite: 100% of getting one

Examples of recipes using Mechanic:
Mechanic + Devilish: Potion of Silver coating (Silver weapon spell, 5 min. duration)
Mechanic + Demonic: Potion of Cold iron coating (Cold iron weapon spell, 5 min. duration)
Mechanic + Madness: Potion of Byeshk coating (Byeshk weapon spell, 5 min. duration)
Mechanic + Draconic: Potion of Force weapon (Applies +1d6 Force, 5 min. duration)
Mechanic + Gigantic: Potion of Adamantine coating (Adam. weapon spell, 5 min. duration)
Mechanic + Undeath: Potion of Harm (like spell)

Examples of recipes using Draconic:
Draconic + Devilish: Potion of Fire coating (+1d6 Fire, 5 min.)
Draconic + Demonic: Potion of Cold weapon (+1d6 Cold, 5 min.)
Draconic + Madness: Potion of Acid weapon (+1d6 Acid, 5 min.)
Draconic + Gigantic: Potion of Shocking weapon (+1d6 Shock, 5 min.)
Draconic + Undeath: Potion of Unholy weapon (+1d6 Unholy, 5 min.)

Examples of recipes using Madness:
Madness + Devilish: Potion of Freedom of Movement (5 min.).
Madness + Demonic: Potion of Poison Immunity (5 min.).
Madness + Gigantic: Potion of Disease Immunity (5 min.).
Madness + Undeath: Potion of Restoration.

Using Devilish:
Devilish + Demonic: Potion of Dimension Door.
Devilish + Gigantic: Potion of Greater Heroism (like scroll).
Devilish + Undeath: Potion of Infravision (5 min.).

Using Gigantic:
Gigantic + Demonic: Potion of Wild Instincts (as scroll).
Gigantic + Undeath: Potion of Heal (like spell).

And the last:
Demonic + Undeath: Potion of Death Ward (20 min.).