I really want something like a free for all arena as a safe minigame. It could be really fun and fantastic XP. The area would be on OSRS gold the size of the fist of guthix arena with lots of obstacles, ranging towers. . .etc. It won't supply runes, but it will yield about 60% of the runes you've used.
Exactly like the arrows burst. Now there should be no such a thing as free XP. For in this area, you will need to buy the time that you are planning to play. You could pay with coins, FoG tokens, or runecrafting tokens, along with other member's tokens.
Just to make it fair for the people that have a hard time earning money. Bandages could be provided together with a milder version of the strength potion. The place could be divided into to segments: Area with levels (such as the uncontrolled ) in which you can just attack opponents near your level. A free for each area.
I believe there should be no demand for buy RuneScape Mobile gold rewards as it'd be pretty interesting how it is. For people not to get super fast XP by 1 hitting noobs all day you could get a smaller quantity of XP gained (similar to battling random events).