Scratch

Hashes (associative arrays) are a more general construct than arrays, though somewhat slower.  For a programming language like scratch, it may be better to include hashes than to include arrays, since you don't have to worry about the bounds of the array and you can index by floating-point numbers, strings, sprites, costumes, ...

I like your idea of an advanced blocks section.  I would like to see Scratch evolve into allowing a string to be put into a variable, have an on-screen text field for display,  and be able to import a text file into an array or the variable.  With the high intelligence of the people who have developed Scratch, I can see no reason to stop Scratch at the level of teaching beginning programming.  It is actually ahead of the curve already because it is cross platform and makes a web page.

Array and Hashes are both pretty easy concepts to teach kids.

The "mailbox" analogy works well for arrays.  There are numbered slots and they're either empty or filled with a value.  [If you implement them as hashes, then scaling isn't an issue and the programmer can make as many mailboxes as they want starting at 0, 1, or any other index and count by 1's, 10's, or whatever.  No "dimensioning" or any of that *.]

Without getting fancy, a slot's default value is 0 unless it's been set to some other value.  Testing doesn't create slots, but setting them does.

Hashes I usually explain as dictionaries with a key/value pair.  It's almost never searched sequentially (but can be) and new entires can be written between other entries easily.  The concept is simple enough, but I'm not sure how (or why) you'd implement them in Scratch without full string support.

I do hope you implement arrays though.  There's a lot to be learned from a simple array type.