So your paper design is most similar in spirit to CLAP. I would say that the actual audio/event processing bits are the "easy" part of the API.

> And here VST3, which has a defined model for parameter interpolation, is easier to deal with than plugin formats that do not define exact interpolation formula.

So I'll just reiterate that this is not true for either plugin or host developers and that's not a minority opinion. The parameter value queue abstraction is harder to implement on both sides of the API, has worse performance, and doesn't provide much in benefit over sending a sparse list of time-stamped events and delegating smoothing to the plugin.

> As I remember, plugins may also have output parameters, so it is possible to process parameter data using plugins.

The host forwards those output parameters back to that plugin's editor, not to other plugins. You use this as a hack to support metering, although in practice since this is a VST3 quirk, few people do it. Until 3.8.0 which added the IMidiLearn2 interface there was no way to annotate MIDI mappings for output parameters, which caused hosts to swallow MIDI messages even if they should be forwarded to all plugins. I doubt that the new interface will be implemented consistently by hosts, and now there's a problem where old plugins may do the wrong thing in new versions of hosts that expect plugins to be updated (this is catastrophic behavior for audio plugins - you never want a version update to change how they behave, because it breaks old sessions). There's also no good way to consistently send what are effectively automation clips out of plugins, since the plugin does not have a view into the sequencer.

And most importantly - plugins aren't aware of other plugins. If one plugin outputs an parameter change it is meaningless to another plugin. Maybe if both plugins implement IMidiMapping2 the host can translate the output parameter change into a MIDI event and then into another parameter change. Sounds a lot stupider than just sending discrete MIDI events.

Essentially, the design of parameters in VST3 is fragile and bad.