Anybody using mockup patterns? Expect huge bloat if only a couple of us will do!

In Addition online fiddling with a JS file in the editor and firebuck - edit - reload the browser cache is no longer possible.

What do you mean? When you're in development mode, you certainly don't have to re-compile the bundle on every change. Just reload your browser and work.

  • The bloat of 170KB for each additional bundle.

As @cewing said, use stub modules

  • We may need a fine grained flexibility to load a resource on demand.

See: http://docs.plone.org/adapt-and-extend/theming/resourceregistry.html#adding-or-removing-bundles-from-a-request

  • The process of compiling a Bundle TTW has to become more tranparent and give more feedback.

It's a hard problem to solve. If you're doing heavy JS development, it'll be better to develop those on the file system.

  • The fragility of the resource registry and its flaw to lockyourself out. A single not compiling JS-file in the Plone or Plone-Legacy bundle disables the complete ressource registry. Since one cannot delete registry keys - in the worst case you have to go into the debugger to delete the keys. This is not an obstacle for the developers, only - let an user/admin install a plone package with unclean JS code and the admin ist stuck and has no idea how to fix the problem. Usually unclean JS code comes with none or a broken uninstall registry.xml file.

This should not happen. 1) hopefully the RR always loads and 2) if it doesn't, there is a backup noscript wrapped tag to delete bad bundles. What version of Plone are you on?

Don't get me wrong. I do not like to start a "I hate Plone 5" flame

No problem. It means we're not communicating something because I think we address most of your concerns.