+1 for initiative, points that are nice to be addressed are below.

1. "Python 3.4 with modern idioms" - too Python-specific code raises the barrier. I'd prefer simplicity and portability over modernism. Like how hard is it to port the parser into JS with PythonJS, for example?

2. ASDL specification is mostly offline. One PDF survived, but IR browser and source for did not, which is a pity, because visual tools are one of the most attractive. In any case, it may worth to contact university - they might have backups and resurrect browser in Python (GCI, GSoC).

3. File organization. This is bad:
 Grammar/Grammar
 Parser/
 Python/
This is good:
 Core/README.md
 Core/Grammar
 Core/Parser/
 Core/Processor/ (builds AST)
 Core/Interpreter/
 Core/Tests/
I wonder what is PyPy layout? It may worth to steal it for consistency.

4. Specific problem with ASDL parsing - currently, by ASDL syntax all `expr` are allowed on the left side of assign node. This is not true for  real app. It makes sense to clarify in README.md these borders (who checks what) and modify ASDL to reflect the restriction.