1) add line numbers to this editor?

Because it would simplify the search for errors for instance.

We all just use Edit in Textmate, then you get it all :)

2) to apply syntax highlighting to this editor?

This would mean to change NSTextField to TM's window object.

Of course this would be ideal for the future, but Edit in TextMate is almost as good. (Install it via the TextMate bundle, use via the Edit menu.)