Ask HN: What features do you want in modern debuggers?

I am trying to build a better debugger for C/C++ and hopefully for Python and Javascript. I am not too happy single stepping through various lines of code to see what got executed and what did not. When a program stops at a breakpoint, I think debuggers should be visually able to show which lines got executed so far and which didn't. This would really help in speeding up the process of debugging (especially new programs) What other features do you think must be present in a modern debugger?