Really wish we had a third tier of mount speed in older zones
And make it affect flight speed. I know, I know. I'm killing the overworld with this suggestion, but am I really?
Think about it this way instead. There's a lot of zones in this game. 4 expansions worth, with another to be added Dawntrail. That's a lot of places for people to be spread out across. Sooooo, if you really wanted to crowd the overworld and make it feel more lively, your best bet isn't to try to achieve success across every zone. Instead, you should attempt to funnel activity into the latest zones.
So, after you complete an expansion's patch content allow people to do something that gives them a third tier of mount speed(or make flying faster). My preference would be something like clear out 5-10 fates in a zone, but there's other possible options that fit I'm sure. This will let people conduct their business in older zones faster. Being able to do whatever they want there will allow them to spend more time in newer zones doing whatever.