I think it would work better as an achievement type thing, maybe a daily rotating/resetting thing similar to the skill of the day, such as:
get 5 kills above x wilderness
get 5 kills in x zone
get a kill with x weapon
Get 3 kills on x spellbook
etc
These could either give upgraded emblems or just cash/random items/pkp rewards (Not huge amounts but enough for them to be worth doing)
The slayer task thing doesnt sound awful but slayer is and has always been a PvM thing and bringing it into pking doesn't seem like the best idea, imo an achievement based system would suit pking more.
I know there is already an achievement system but from what i can remember there's 1, maybe 2 PK ones. I think daily resetting PK Achievements would end up with a lot more active pking.