So I'll link this back to an issue on github and the topic where this was discussed before. Just those don't get lost
I agree with Toke in that artists need to see instances and be able to toggle them on and off. Secondly I'd keep any optional plugins in the ui too, so artsts can have some control over formats exported for example. Any other plugins I'd only show in case of problems.
Overal I'd make the UI bigger. Actually I'd say that a version of the one mentioned here would still be the best.
Here's an ugly mockup so take it with a pinch of salt.
Left - Instances and maybe basic info about them.
Middle - Optional plugins and those that developer specificaly marks as visible. (or the other way. showing all and giving option to hide from UI when writing plugin)
Right - Additional publish info. Ideally Each of of these might be a plugin, so we could populate it with what we want. This could be shared the same way as plugins now. This would require giving developer an option to choose how such plugin would show up in the GUI (image box, text box, button as other plugins...) This is probably a feature on it's own that would need to be discussed, but hopefully you get the jist.
In case of failed plugins, we could either roll them out from the bottom, just switch to the overview tab, that could look the same as publishing now, where these could be dealt with.