My preference would be removing the "roulette" part. Let people pick which bonus dungeons they want to do. Some will be more popular than others, sure - but that will fix the issue of people leaving just because it's content they don't want to (or maybe even can't) do. It would be nice to see the amount of players required for instances too, so people can choose to go somewhere where they would be most useful.
The story primals would need to be split into their own category, maybe even split the remaining low levels into 2 categories. And for the instances that never seem to fill? do the same as they do for tanks/healers - you pick that one, you get a little extra bonus.
Not a perfect solution - I doubt there is one - but it would keep a far larger group of players content, and keep people in the lower level dungeons so newer players can progress without long waits.
