Flexible I/O
Golfscript cannot at present be used for interactive programs. I propose some functions for explicit input be added (i.e. readline, getchar and friends). The interpreter should see if the program uses these before running it.
If the program does not call any input functions, the interpreter should act like Golfscript normally does.
I wouldn't expect the interpreter to detect input functions in eval'ed code generated at runtime, but if it somehow can do that, kudos.