Beastmaster guild is quite cool but unfortunatelly not popular. Probably
because isnt too much powerful and riding a mount has his problems if you
plant to party or entering some areas.
I have some sugestions:
Some mounts may give bonuses to 'Ride through' and Ride underground skills.
Make some quests about taming some especific different mounts so they give
some bonuses.
Allow selling (and buying) mounts in a market at the beastmaster guilder to
encourage the taming of hard mounts and make some cash.
Well, just some ideas

++Coyolxauhqui