Someone had posted a suggestion once about having an 'invisible' mount that would give us a speed increase but with no visible mount so there would be absolutely no visibility block with the mount. I liked the idea somewhat, but am expanding on that idea with my suggestion.
I would love a mount that gives us 'winged feet', like the Greek god, Hermes had. The mount would be visible, but with no visibility block, and it would give us the speed increase just like the other mounts. Our characters would not 'hover' above the ground, since these are not your traditional seraph or dragon wings, etc. A neat animation effect would be to see the 'dust' rise around our feet and ankles just as we take off running from a stand still position. :)