I always thought construction would be a lot of fun in RSC. There actually was a 'Carpentry' skill way back that was on the skill menu bu never fully implemented, they were going to have player owned houses right on the main map (no portal), but due to the games popularity surging, they took it out.

To be honest i think that would be the only fun skill added. Runecrafting would destroy the rune economy of RSC and to be honest, i never had fun with the hunter skill in the official game.

Just my two cents, house building would be neat, but i know the admins are busy with slayer right now (and wondering what else is going on with the PUBG type mini game) so it may be a while before they even consider it.

Building and decorating your own house with house parties would be cool. Maybe while being at it, go for a new continent with new skill requirements too, which of course can create more quests, even perhaps some new slayer-related bosses with not too OP drops, but enough to have somewhere else to level up to.
Maybe even an actual storyline with custom features.