Battle Square from the Gold Saucer in FF7. Kind of like how you obtain assault points now, you can get points for battles to obtain gear in solo battles based on race, job, level, and gear (maybe certain gear is not allowed to make playing field more even). The points can be accumulated for specific battles and some gear can only be obtained by major accomplishments such as beating certain difficulties, completing the entire mob selection, and other various circumstances. Allow players to chose increasing battles difficulties as the competitors win previous difficulties and final battle is extremely difficult to the point of absurdity, but gives access extremely good equipment.
I like the idea of allowing KS99 to drop abjs., but as it stands, that would make the items extremely common and worthless. It would be like the O.hat was 2 years ago, a great piece of equipment not everyone has, but today is considered standard gear. I don't like the idea that in order to obtain gear, time is required in vast amounts. I don't mind spending weeks or longer obtaining an item, but spending hours of my time at specific times is extremely difficult for the average player who works for a living. People are willing to do sky because they can do it at any time, but the three kings don't work around your schedule, instead you have to work around theirs. I wouldn't mind another version of the King fights in a BC that is hard to get access to and very difficult to win. Maybe introduce another form of the BS or KS make them less common drop or need far greater amounts to do the fight. Make the fight instead rediculously hard so that the items remain uncommon and the fight is all about skill. No, manaburn, no easy strategies that any idiot can follow. Soemthing that really makes the game a challenge even for the players that are used to all the difficult challenges already in the game. Possibly even the dynamic mob like others have said.
I'd also like to see a bunch of random NMs. NMs that don't require camping, require large groups to do, have decent drops items, and act/spawn differently from previous H/NMs. No not pop NMs. Maybe a HNM that has repop of 15 mins yet is extremely difficult and drops good items. To make the good items worthwhile and not just common, the drop rate is very low much like the thief knife. I also like the idea of wandering H/NM that appear in different zones (open areas and not dungeons would be best), but at the same time not in common low level experience areas where less experienced players would get randomly pillaged. It gives rewards to those able to mobalize members at various places quickly and at random times. And dynamic mobs is a huge idea many have already said. For many the same old mobs and strategies are boring especially when the method has already been made and determined for you.
like gold Saucer from FF7 
