Reading this again I like your idea more than my last post lol...
I would do like a Zuk Challenge Fight Cave thing like you said but, where you do a wave challenge and then fight with whatever you have left in supplies, when you finally meet up to the same location. I could see this working more as a 1v1 though. Players would have to meet a minimum/maximum risk requirement in order to play, where if you die, you'd lose everything you've risked (like t70 weapons/armour only, for example).
The interesting mechanic I would add to it would be Arena Abilities exclusive to the area and can fluctuate between sets so Jagex can edit and alter what's in and out of the game. So, one set would be up for a given amount of days before a new set comes in. You'd have an outside source in-game that offers new abilities and he would be like an NPC who decides what can and cant be used, like a creator of abilities (so people blame the NPC not Jagex lol <3)
That's an interesting idea
200m Dunge, Rc, Mage and invention accomplished, + 120 Constitution/Slayer/Range/Summoning/Woodcutting/Mining/Defence/Strength and 110m fishing, got comp/MQC and 200m Fishing/Woodcutting/Mining as well as 120 herb/Farming/Cooking most 120s just Agility, Theiving, Hunter and Prayer left
cheers
200m Dunge, Rc, Mage and invention accomplished, + 120 Constitution/Slayer/Range/Summoning/Woodcutting/Mining/Defence/Strength and 110m fishing, got comp/MQC and 200m Fishing/Woodcutting/Mining as well as 120 herb/Farming/Cooking most 120s just Agility, Theiving, Hunter and Prayer left