To go off of what many others have said, I really do think they should add achievement mount skins. Not to replace their current mount adoption system, but to compliment it, to add to it. Because it is a company, and it's a free to play game so they do need sources of income and profit. But I definitely think they should add achievement mount skins to reward their players for putting the time in to play their game.
What would be a cool idea is if they had area exploration achievements that you can get race themed mount skins for. Like a Norn griffon, or an Asura Springer, and etc. They could make multiple of these achievements divided up in clusters of zones that surround each race's main city. They can even make some requirements for an achievement overlap with another if a race shares a zone with another. Since I know Asurans live in close proximity to the Sylvari, and etc.