With the War Within, we will be able to gather transmogs for all armor types on any character.
Considering a lot of people spent time farming a set for their own class, and in the meantime got drops for all other classes that were basically useless at the time, it would be cool to unlock transmogs for a tier set for all classes, once one character gets it for their own class.
For example, I farm the Tier1 Might set in Molten Core on my warrior, and when I gather all items in the set, I unlock the transmogs for all sets in Tier1 for all classes.
Additionally, it would also be cool to have the ability to farm only 1 difficulty in later expansions - for example, I collect the Emeral Dream raid mythic dificullty tier set for my warrior, and all lower difficulty sets would be unlocked for me (Heroic/Normal/LFR).
This obviously wouldn’t go the other way around, i.e. I collect LFR set, of course the higher difficulties won’t be unlocked for me.
So combined, if I farm 1 set for Mythic difficulty, it would unlock all sets for all classes for that Tier, across all difficulties.
If I farm 1 set for Normal difficulty, it would unlock all sets for all classes for that Tier, but for Normal and LFR difficulties only.
And it would also be cool if this is applied not just to raids, but to dungeon sets, and all other sets that are in WowHead’s database (the “unofficial” ones, they can be made official as well)