Lowering the rewards but increasing the rate at which the marks can be hunted will keep the interest.
Right now Rank B marks are on a 1 hour timer. 5 allied 30 myth 10 sold. per mob / per hour. <-- hunting single rank B mark
Or as I proposed: up to 20 allied 20 myth, 4 sold per hour. <-- if you were able to kill each time that Rank B mark spawned.
Now if you jump from zone to zone hunting a Rank B mobs your rate of reward can go up as you have many more times the opportunity to encounter an Rank B mark.
Greatly increasing the rate at which a lower rank mark spawns will keep small groups hunting in the zones, or hopping from zone to zone as the chances or contacting one increases.
Rank A marks are alive for less than 30 seconds once they are engaged during prime hours. These marks have very long spawn times. Increasing the spawn rate and the difficulty will increase the opportunity for larger parties to gather and confront the mark.
Rank S marks right now are rare, because spawn conditions/methods are still unknown. Speculation has it that they maybe a 24 hour spawn. When they have spawned, they have been defeated in minutes. These two things greatly reduce the opportunity for anyone to ever encounter a Rank S mark.
Similar to I've been playing this game for almost a year and I've seen Odin 4 times and Behemoth once. I'm sure NMs will be more plentiful than Odins and Behemoths. But Myself and others don't want be able to say hey lets go hunt some NMs today and actually be able to do just that in a 1-3 hour period. Not having to dedicate our entire play time running around zones, glued to our chat log and teleporting at the first report of a NM sighting in hopes that 1) we can zone in before it's dead. 2) get to the mob before its dead. 3) get a hit or two in to at least get some credit and 4) people are kind enough to wait a few minutes before engaging the mark so the max number of people can get full credit.