I think it would be really great if the harder dungeons had a permanent mount drop from them (this isn't talking about the instance dungeons that you have to buy btw because AFAIK those already have perm mount drops).
The mounts would be specific to and fit the theme of the respective dungeons they drop from. This isn't a crazy idea BTW. A LOT of other RPGs have mounts drop from dungeons/raids. In fact, I'd go as far as to say it's the norm for games like Wizard101 to have mounts drop from dungeons.