As with everything else, mount speed increases wont be able to continue (inflation) forever. Another idea would be to give mounts specific enhancements.

A few ideas i have heard would be to have:
The floating mount from SL (forgot name) - hover over water

The giant mount for SL pre-order - no fall damage [since he has big *** arms and should be able to "run down mountain faces"]

A pvp-mount which you purchase at high rank/lvl - Granting more valor/veng and can fight while mounted [the speed could be reduced to make it so you can't do drive by assassinations.

Raid mounts at high lvl/currency: to be used to give stat bonuses for open world raiding/xp (new hunt rifts/etc.)

The only downside is i would expect that everyone would be equipping mounts for the bonus and fighting without one would become obsolete. This could be combated by preventing certain abilities or a reduction to a certain stat when mounted to level the playing field. Making it a strategy for certain raid members to be mounted (maybe casters for mana increases) and fighters on the ground unmounted (so they don't incur some kind of debuff or something).

There are a ton of mounts in the game currently and more are being introduced, whether for higher speed or merely another novelty item. I feel a lot of creative additions could be tacked on to add another element to character planning.