Hello there, I have a few ideas for classic:

capture the flag/castle wars:
Two islands and island hopping to get to each area, and we can have items that spawn, kinda like brwn's god armour that hes making from classic kbd(maybe players can be invincible for x amount of time, kinda like sonic the hedgehog with stars).. Players can can get weapons from the armoury at their castle/food/potions and special weapons can spawn for x amount of time.. so basically players start with nothing, they get what they need and fight eachother.. stronger weapons can spawn in the middle of the map so its a race to get there.. lots of things we can do, make towers for archers ect..

rsc doom:
I thought this was an interesting idea, doom, just like the shooter game, where theres maps that can be created with spawning items and who ever gets to them can use them to fight eachother, with multiple floor plans so people can hide or fight..

rsc battleship
This idea got me excited, what about boats fighting eachother and moving across the waters? they can range eachother, and fight to the death til their boat is to heavy damaged to fight, players can get more cannon ammo, on certain islands, i dunno how big the map would have to be but i think it would be pretty fun..

rsc pest control:
Yes just like rs2 theres a place you can go with npcs that you can fight and players, to gain xp and you can earn certificates to get cool new items..