The issue with adjustment layers is that GIMP does not yet have a mechanism for continuous filter/plugin evaluation and it may not yet be fully thought out: so what does the layer mask do in this case, just a wet/dry mix, some other plugin-specific parameter per pixel? Also, if we're going this route it might make sense to have adjustment layers attached to a parent layer where they apply (as descendants), and then be able to reorder them within. And if we're going that route, they need to get the full hierarchy model for layers in there first.