In case you need your heart broken, there's evidence to suggest Scratch Team were seriously considering allowing blocks to placed inside of variable dropdowns.

Evidently, that fell through. Something didn't work.

Just looking at Snap! for inspiration-- wait a minute..

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Global custom blocks have been released for Unsandboxed.

Full changelog in the article.

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An update has been released for Unsandboxed.

Changelog is in the article.

in the next build

Currently working on a "Flash" block style.

So I can say for certain that Scratch 4.0 will depend on modern Blockly, which for now is being held together by the old GUI code from 3.0

This is why scratch doesn't add new blocks FYI. The sensing_online block was added out of irritation rather than care.