I think adding a Duo Elite slayer would be very fun and interesting, to avoid abuse maybe can only team up with people within 5 slayer levels, or maybe limit gear used for example you can only use elite slayer gear, and/or lets say the lowest slayer level of the duo is 10 maybe maybe limit the tier required to 1; 20 2, ect. Another way would be to add hard mobs that require more than a single person to complete. The final way I was thinking was to make it both people have to do atleast 1/4th hp of the mob. (Doesnt have to be exactly this ofc just spoutin ideas :D)