In a way, this ability already exists if you don’t unlock Pathfinder. But if you have Pathfinder for older expansions and want to be on the ground, I suppose I could see wanting this option?
For RP I can see the benefits, if I think about it more, too. I know I get annoyed with druid auto travel, so I can’t judge anyone for an option like this.
I vote for both: option to make most ground mounts fly and option to make flying mounts ground.