To implement implicit parenthesis is possible to add a parameter to macro definitions to tell the reader how many parameters the function takes, so that it can add an opening brace when it sees a macro name and it closes it after n arguments. I've done so in a toy lisp interpreter I'm writing, but after a while i removed that feature because i didn't like implicit parehthesis, when manipulating forms within macros you have to remember about implicit parenthesis and this makes macros less natural.
Arc really needs a FFI. This would make possible using graphical libraries such as GTK+. I know Arc is web-oriented, but for some type of applications a conventional GUI is simply better.
Implementing strings as list would be rather slow and memory consuming: you would have to use a cons cell for every character.
Implementing strings as vectors and making them visible to the user as lists would be interesting.
It doesn't matter how they are implemented (they can even be cons cells implemented as closures), as long as you can use the ordinary list functions to manipulate them. But this is the direction Arc is headed to anyway.
Using M-expressions would reduce the regularity of Lisp, IMO. But mathematical expressions are more readable with infix notation. I think it would be nice to be able to say (math 8+9(sqrt 67)) instead of (+ 8 ( 9 (sqrt 67))).
