Scratch

This has been suggested so many times, and some mods have it (Panther for one, and I think Slash has it).

Do you know how to add blocks to scratch?

If not, I suggest looking up, 'shift+click+R' on the forums to see how to change scratch.

Then when you can successfully get the the browser, you can add new blocks to scratch*.

The blocks most likely will come from Your Local Block Library

* = The blocks will not work online or on anybody else's scratch, unless they also have the block in their Scratch version