Hi
Slayer needs CPR. Nobody runs it if possible because of the low XP/rewards/interest.
Here are my suggestions:
XP points for each kill - starting with 1XP at lvl 1 and scaling up from there
Triple the XP for each slayer level (i.e. 10, 25, 50, 100 etc.)
Add many more beasts to every slayer area except The Vale, Amranth and House Cannith
Have kills drop plat and crafting ingrediants
Make slayer areas have multiple parties like Mabar
cheers