Don't want to snipe at all the rangers out there, but personally I feel like rangers tracking ability almost completely ruins the class. Rather constantly risking the dangers of the wilderness as you would imagine a ranger would be doing, they can simply kick back and "track" people across the world to make their money. I think it would be more interesting to see a ranger use a track skill similar to the ninjas, giving direction moreso than location, with a large twist however, make it impossible to "stealth" from a ranger. This would increase their value as a party member and get them off their lazy asses. xD
Removing the "sales pitch" of the ranger and throwing in a game changer I think is the right course here, as has been suggested, it opens up a whole new need for teamwork when tracking.
An interesting idea:
I think a cool tracking skill would be something along the lines of a summoned slime/wolf/baby zombie/etc that is specifically scented towards a player. The ability has the plus of still being a type of track that can be sold, with varying speeds and options for implementation across classes. The track would be able to be "killed" and in most cases (perhaps another option) is going to be easily detected once nearing it's mark. On second thought, this may not work well, as it would be hard to direct the summoned mob across/around water and certain other obstacles.
I think it would be interesting to see a more interactive tracking system, however. One in which many classes can acquire a vague idea of an enemies location, and only a select couple have aggressive tracking abilities.