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It would be great to autoindent next line when writing code blocks (that start with 4 space, for example).

Sounds like you're looking for tabIndent.js

This breaks simple things, like trying to do multiple lines of four-space indented code formatting, for example.

It also breaks trying to type out quotes and nested quotes.

It was one of the first things I tried and it immediately disturbed the normal flow. Markdown normally allows for a "one line per sentence"-style. If you are used to that, the current behavior quickly becomes annoying.