Syncing widths and sidebearings
The ability to sync widths between the main layer and the bubble layer is something that could be useful.

Whenever the RSB changes, the relationship between the glyph and the bubble are maintained but then their widths are out of sync.
But the real issue occurs when the LSB is changed, where it is immediately obvious that the glyph and bubble are out of sync.
In this case, the process becomes difficult because the LSB changes 12 units then I have to manually change to the bubble layer and also change that 12 units to keep it in sync.
I think a major advantage of BubbleKern is the ability to work on spacing and kerning at the same time so keeping the bubble shape in sync is crucial.
Not sure if this would be something specific for the plugin or could be a more general feature.
And on a similar topic of keeping the bubble and outline in sync, it would be useful to be able to select and move nodes from across the layer. I raised the idea in this forum post for the Select All Layers tool to work with non-master layers.