I know the todo list is probably long. And I know everyone probably has a different wish list, or at least a differently ordered one.
Here's mine, sorted with the most-wanted first ...
 User defined commands written in APL, and invoked with a bracket. This has its own discussion in a separate trail.
 A new system variable (⎕ln ?) with the names of all the called functions, corresponding to the elements of ⎕lc.
 A new system function (⎕cmd ?) to call an external OS command. Returning its return code is important. Collecting its standard output and error output would be nice, but much more complicated, and much less important.
 An interrupt button on the session manager.
 User defined function keys in the session manager.
No doubt there will be others if this discussion gets rolling ...