Currently the only players allowed to choose their own slayer tasks are eternal donators. My suggestion is to allow all players to choose their own slayer tasks for a x amount of points decreasing as your donator status goes up. I got this idea off the current OSRS DMM tourny where they can choose their slayer task with a sigil for 30 slayer points. My personal idea for ranking listed below.
Non donator-not able
donator-300 points
super- 260 points
extreme- 225 points
legendary- 150 points
uber- 100 points
master- 75 points
immortal- 50 points
eternal- free
Please keep in mind this is for a single task, I don't think it'd be too OP for the amount of points this would cost vs players just going and fighting the desired monster they want. Also I'd like to add that most players can just skip tasks for free until they receive the task they want by cancelling their streak, this would just allow some players who want to keep their streak to pay points to do so.