pretty much osrs slaying system which I like.
My suggestion:
The more popular tasks, which give good xp/hour or/and have a very good drop table need to get unlocked first (nerf to slayer as it is now).
With the ability to block tasks you do not like permanently, slayer becomes more enjoyable than it is now (buff to slayer).
You can also unlock the ability to extend tasks. However, after recieving a slayer task, you will be asked, if you want to extend that task for some SP. Bosses will cost more SP to extend than other monsters and the extension varies depending on your slayer level as well as a little bit of RNG (SP sink).
This will turn SP into indirect slayer xp (better than adding xp lamps) and makes the skill more enjoyable.