Slayer is a fun, rewarding original skill that, for the most part, doesn't need many changes.
However, one change that I know thousands of players are yearning for is the coop component.
There are definitely times both myself and a friend want to train slayer. Unfortunately, when we get that task, we are more than likely going to opposite regions of Gielinor. That's honestly something Runescape needs more of: cooperative content. This would be an excellent start to getting back to that.
I can understand that some will mule and try to let the other person collect on the task, but there has to be a way around that; there's gotta be a way players who wanna slay together can do so without the others ruining it for them. Perhaps XP could only be given to the player that kills the monster, or something along those lines.
I do believe this would be an excellent addition to the game again; and would promote more cooperative play amongst players. I can't see anything wrong with two players slaying monsters together and both being able to receive the rewards. It would make an already great game even better.
Thank you for taking the time to read this.