Nothing fancy, just any gear slot adding bonus xp (and nothing but bonus xp). Pocket preferred.

Account-wide. This is important. Every new character gets one in their bags. People are altoholics.
Exclusively when purchased from the website store for real money, not store points.

I know three friends who made a new account when the exceedingly generous store codes for free areas were in play.
I'm sure this addition would trigger a good amount of sales and also in part satisfy the complaints I see from a VIP perspective.