Long time since I have made any contribution, but with the new Transmogrifier NPC being introduced in patch 4.3 there is a big opportunity to make some gold.
Fun/nostalgia BoE weapons will most likely go up in price. Items such as:
Teebu's Blazing Longsword
Blade of Hanna
Sulfuron Hammer
Core Marksman Rifle
Krol Blade
Dark Herring
Generally items which has a symbol in WoW/items that would look fun, will most likely rise in price.
Weapons can be customized as shown here.
A blue post stated it will NOT be possible to change your weapon into a legendary. (source)
So try to take adventage of this by buying fun/rare items on the auction house that will sell well once you can change the weapons look into it.