For reference, this is what I use to develop my plugins, it's a managed wrapper for creating plugins in C#, and provides a bridge between VST and my interface, called SharpSoundDevice. I develop plugins for personal use, and my choice of C# is not up for debate here, I'm fully aware of its limitations :)
To me, the biggest problems are:
1. Docking the UI inside the host. This is just an awful way to do things, as the host can intercept (and break) lots of event bindings in your UI. Notice how the scroll wheel and typing are broken in every other plugin in Ableton live? yeah, I mean that kind of stuff. Plugins should have its own window, but then of course, you end up with a problem where they should ideally always be on top of the DAW window, etc... This also means you have lots of platform dependent code for interfacing with the host. (I do my UIs in WPF these days, with some magic to dock the wpf panels inside the win32 window, it's ugly and I wish I could just handle it all inside my own application without worrying about docking windows within windows, etc. Solutions for developing plugin guis exist (like JUCE and VSTGUI, ugh), but it's a restriction on creativity and possibilities for developers (esp. for newcomers who are not familiar with how VST works). Wouldn't it be nice if we could just write "a desktop application", in Qt, WinForms, PyGTK or whatever, and it would just work? It can be done, but it requires someone to sit down and think. real. hard. which leads to my next point...
2. Process isolation... is hard.
Wouldn't things be nice if we could just run each plugin as a separate process. Well, you can, but you endure painful context switching. For multiple channels in parallel, it's not actually so bad, because all modern DAWs are multithreaded and can make use of the "lost time". The problem comes when you have 20 plugins in series, and each of them adds context switch latency of 10-15 uS, adding up to half the available time you have to process the buffer :) Still, some DAWS (like Bitwig) claim to run each plugin as an isolated process, I'm curious to know if they just say "fuck it, live with the added overhead", or if they have some magic solution to mitigate the issue.
I applaud the effort, but there is one more issue remaining:
3. Nobody is going to adopt your standard verbatim. Which means people will be creating wrappers, which means inheriting all the warts from VST/AAX/AU plugins. This is what annoys me most about my own interface. It's a relatively clean .NET interface, but I end up having to cover it in a layer of grit to stick it into a VST host :)
...yet I have not given up on building a universal, platform/gui independent, process-isolated plugin standard! Currently experimenting with named pipes and even Standard IO to define a common interface between host and plugin, which should allow running the gui in a separate process, and allowing you to write an audio plugin in any programming language that supports stdio... :)
Thanks for your message!
For the UI docking, it comes has an extension for clap. First there is the gui extension, which just has open()/close(), and then comes an other extension: embed (host), embed/win32 (plugin), embed/X11 (plugin), embed/*. So embedding is available only if the plugin support it.
It would be really nice to have the UI done with Qt or Gtk. Yet they're not fit for such plugin UI (because of the global variables and the main loop, ...).
One of my friend (phant0m) is starting to write a UI toolkit which does not rely on global variables, and should be convenient for plugin UI. I think that we can achieve pretty cool UI with a 2D scene graph, with a vector backend like cairo, and some event handling and windowing abstraction. So work is in progress here.
For the process isolation I like a lot the idea, but if I can have bug free plugins, use them in the same process and benefit from a noticeable performance gain, I prefer "in engine" hosting. Also this is the new option of Bitwig: "only as a bit bridge".
Also the one plugin/one process idea is already possible with jack, right?
For the adoption, it is a big challenge. Let's see how it goes ;-)
I hope that I did not forget to answer one of your point.