The current inbuilt one will allow step by step debugging ? Although it can be a bit hard to read sometimes and user interface could do with a polish. What were you thinking of adding. If you've not seen it it's covered in one of the Swift tutorials.
Although one of the thoughts I had was to allow the stepping process to highliight the lines in the source code in Swift, which would be make step by step debugging much nicer ! But that would involved me going into the BeebEm code and setting up some sort of communication between the two which I didn't fancy doing
