General implementation

Execute lisp source file as program
Use hash maps for variable lookup?
Tail call optimization
Define functions with let
Pretty print parse errors
Floats, or some other non-integral numeric type
void type
Variadic lambdas
Square bracket lambda syntax
Slashdash comment syntax
User exceptions

Image processing

Change stroke width?
Translation, rotation, scaling?
Save and load images

Code maintenance and refactoring

Handle draw errors more cleanly, rather than using std.debug.print
Code deduplication on primitive impls, particularly arithmetic and draw ops
Type reflection to automatically generate canvas_runner.Message type and implementations
Refactor Evaluator.lexical_settings to avoid code duplication and code bloat

Bugs

Conditional declaration
range sometimes does not include lower bound when abs(step) > 1
Symbols starting with _ interpreted as integer literal

Outside the code

Documentation
Screenshots
Example programs