I have a few ideas:
Disposable tents with 20 charges that cost ~25g per charge when you don't want to hearth to an inn for whatever reason (say you prefer grinding for a level or two and decide to farm a rare pet as you level, things like that). They work just like an inn, so you don't take a penalty in gaining rested xp.
Ground mount speed increase of 30% for 5000g. With the increase in no-fly zones, I think this would definitely be a successful gold sink. 30% to make it attractive to paladins and death knights.
If someone has the Loremaster achievement or completed a particular zone, a vendor could open up and start selling the quest rewards for approximately 100g - 150g. The amount is high to discourage enchanters. The reason for this idea is a lot of people played before transmog was even a glimmer in Blizzard's eye and just vendored the rewards.
Learnable flight paths. How it would work is that the main character can buy Fogg's Atlas for 1,000g - 5,000g per alt in order for them to learn the flight paths, which only open to level appropriate areas as the character levels.