I think the best way to handle targeting specific slayer tasks would be to make me a list of the most desired tasks, and you can choose the task and have 1 in x chance to get the task instead of a random task like usual. So for example, you choose a kraken task. You have a 1 in 4 chance of getting the kraken task or a random task. The chance could be a fixed number or change depending on your donator rank, such as opal being 1 in 8 and the highest tier being 1 in 2 or something.