10UTB is good, maybe have as challenging a course as you can, an event needs to have a distinct feature.
we have:
3x 10 under the ben/ kirroughtree/ moray, these are easy, summer enduros.
the strathpuffer. 24 hours in the north of scotland. in the middle of winter.
mayhem,
relentless 24.
lots of others.
anyway, what im trying to say is that although these courses are all very good, the event organising is good etc etc. you need to find a gap, something undone.
i can only think of a few ways of doing this. course design: a brilliantly technical course. money incentive: attract pro's.