Flying will ALWAYS be desirable, because it's super convenient. You can chill out, put yourself on auto-run, access otherwise inaccessible places, the list goes on.
But there are also a ton of mounts that, let's face it, just look GOOFY when flying. And while I appreciate that at least they CAN fly, I still think it would be great if there were some utility in running around on the ground.
Now, I *THINK* there might already be some form of this in the game already? I know there's a "Mount Speed" window with a bunch of stars in certain locations, but I've never been led to believe it was much of a boost.
What I'm personally suggesting is that, while flying is perfectly fine as-is, that "grounded mount-speed" be buffed up CONSIDERABLY. Not warp-speed or anything, but if flying is like 150-170% movement speed, then grounded mount speed could be like 200%. You might still opt for flying, because it's just easier and you can avoid stuff, but if you're REALLY trying to sprint somewhere, being on the ground would offer some benefit.
Maybe then, I'd be more excited to bust out my SDS Fenrir, which just looks kind of doom when flying, imo.