My suggestion is VERY SIMPLE and quite easy to implement.
Add an NPC to dalaran than allows access to the auction house. On the very first WOTLK server I've played on the AH was available in dalaran at the spot where the portal to wintergrasp would be (Horde side).
Benefits to this: Everybody could use the auction house without going out of their way, mage players wouldn't be trash talked, bashed, and flamed for not giving portals for free, and overall more activity in the auction houses would happen.
Not much use other than some convenience, but then there's other things like trainers in Dalaran, they'd also be useful but we gotta draw the line on custom content.
There's AH for Engineers, that's enough.
mage players wouldn't be trash talked, bashed, and flamed for not giving portals for free, and overall more activity in the auction houses would happen.
No mages are trash talked. Opening portals costs money. It is a "service" they can chose to provide. Also, a lot of mages will just open a portal. If anything, you aren't obligated to tip, but it is nice when you do. On Icecrown I always tip 50g and on Lorda 10g as its somewhat correlated to the economy.
Dalaran has an auction house that is only exclusive to engineers. Adding one available to all would remove that unique perk for the tinkers and significantly devalue the profession.