As I recall from long ago, in APL2 it was possible for system programmers to write (often as a model) new code in APL and have it compiled into the interpreter to act similarly to a system function. What about adding that as a feature in APL64 so that new language features could be modeled and debugged as APL code and eventually replaced with a C# version after it has been proved to work well, or even left in APL as a permanent language feature if it's fast enough.
Such a feature might be made available by allowing external DLLs as language extensions so that users could write and test their own software as "external DLLs", perhaps connected as special quad-names (such as ⎕X... or similar). Or a similar capability might also work within the concept of compiling APL code into MSIL/Assemblies to do similar things (though it would probably require a separate ⎕NA connection rather than being built in as a native function call).