Lordaeron - Link both factions' auction house
I suggest linking the Alliance and the Horde auction houses of both wotlk realms.
I put Lordaeron on the title because I don't know if Icecrown has linked auction houses. If it doesn't have then both should have.
Prices will be better adjusted, like in any free market economy: more trading volume is better.
Will improve item availability and more competitive prices. Better buyer satisfaction.
Will have a larger consumer base to which sellers can sell which means more selling opportunities. Better seller satisfaction.
Gives a feeling of the realm being very active.
Some limitations, however, could be put in place.
Like not allowing an account to buy it's own auctions if done with a character of the opposite faction.
And maybe not showing the name of the seller or the buyer (when it comes in the mail) if the seller/buyer is from the other faction. Could set the name to "Unknown", as this wouldn't break any addons that may make use of this auction house buyer/seller name and would be a good "fit" for role-playing purposes.
On retail nowadays the auction houses are linked between factions, blizzard made a good decision in this aspect, I suggest warmane do so as well.
I remember some years ago the wotlk realms here had linked auction houses and it worked pretty well.