I think there should be an area that has all of the slayer assignment monsters that you can use to do your slayer tasks. Probably only spawn a few of each monster and spread em out a bit. And maybe some kind of requirement to get in, such as like 1M cash or something simple. This would be an easy fix to the missing slayer monster problem and it wouldn't require a whole ton of effort since you could spawn all the monsters in the same area.
Or you could just spawn them all in one place and forget the requirements aspect of it if you wanna be really easy with it.