Next idea: "Private Globals". This is a ripoff of the concept of static variables in other languages. Although it doesn't really add new functionality, I think it would considerably improve organization and readability of code. Basically this implements a private function variable that retains it's value globally/after the function exists. This could be use for factory functions and similar. I'd think something like "GLOBAL DEFINE x" to declare it.
For instance, you'd have:
FUNCTION getGlobalCount()
GLOBAL DEFINE counter
LET counter = counter + 1
RETURN counter
END FUNCTION
...
MAIN
DISPLAY getGlobalCount() # 1
DISPLAY getGlobalCount() # 2
END MAIN
The main benefit here is that the variable and its usage are kept together if it is managed by a single function. As such it's more of a readability assist; the behavior would be no different from declaring a module-level private variable with the added protection that the compiler would prevent multiple functions in the same module from manipulating the variable.