For my project, I needed to do some automatic manipulating and displaying of Expressions. In particular, I would get the textual representation of an Expression, and sometimes I would "simplify" some of them (not in place, only for displaying purposes). I found that this simplifying step came across some inconsistencies in the getCompileTimeConstantValue code (where it got modules from, and whether it was null safe), and the Displaying required me to basically copy and paste the existing ExpressionPresentationUtil class, and patch it with special cases.
So, my proposal (and I realize this is a big change), is to make those behaviors aspect oriented. So instead of having a "isConstantValue" or "getCompileTimeConstantValue" methods on the Concepts, you'd have some other extensible mechanism to look up instances of "ConstantValueChecker" classes, "ConstantValueCalculator" classes, and "ForDisplay" classes.
Ideally, any language designer extending baseLanguage could add new instances to that lookup mechanism, and override the results depending on context.
I really like how you guys have made so much of this system aspect oriented. I'd just like to see it keep moving that way.