You could make an option to choose a task for slayer points. Slayer master would give you an option to choose 1 from 3 random unblocked tasks (with random quantities). Slayer point cost should be equal to skipping task cost, imo.

Example
Choose a task:
34 Ghosts
22 Green dragons
66 Ogres

It serves 2 purposes:
1) Ppl get more favorable tasks (loot, afking etc.)
2) New players wont quit slayer and server cos they got 4 unicorn tasks in a row

When implementing, make sure ppl can't have the option to choose multiple times without paying slayer points.

Also, need more ranging spots to new slayer areas.