Slayer is definitely one of the most popular skills in the game- and since the rework, a TON of the common slayer monsters have been packed.
This is pretty annoying for everyone, and discourages some from doing slayer - so, I think a solution would be a Nightmare-Zone style Slayer Instance.
To elaborate, this system would be similar to the nightmare zone in OSRS, but instead you pay a set amount, per hour, to fill your instance with whatever slayer monster you choose.
To balance this out, it needs to be somewhat pricey, I was thinking baseline 3m, then the donator instance discount would come into play after that.
This has lots of benefits, for one, it encourages people to donate. It's a great money sink, and it lessens toxicity from people crashing at slayer training spots. The only con I can think of, is that it'd be very afk and some people would afk train there for a long time, but I think with the instance price, it would discourage that a lot.
The NPC could be found near the slayer master in the Donator Zone, and you can just ask to open an instance. From there, he'll ask which NPC you'd like in your instance, and when you select that, you are then asked to pay 3m for the hour.
I think the instance should hold 6-8 of the requested monster, that all spawn near the middle and are aggressive.
Other options are non-aggressive instances for 1.5m an hour, or a donator only slayer zone with a lot of common monsters with no entry fee.
I think using the Nightmare Zone instance would be perfect for this due to its size, and what the overall idea is.