> For example, I would be shocked if it wasn't absolutely trivial 
> for the current implementation to add auto-indenting following 
> a colon. That feature alone would be a win for usability.

That would be a non-trivial change in Windows. I think it's at least possible using the high-level console API. It could be implemented with the pInputControl parameter of ReadConsoleW in combination with WriteConsoleW. This is how the CMD shell implements tab completion for file paths. That said, many of the proposed UI enhancements cannot be implemented in Windows using the high-level console API. 

IPython used to depend on readline. (5.0 switched to prompt_toolkit instead.) In Windows this was via the pyreadline package, which uses the low-level console API via ctypes. pyreadline is apparently abandoned (last updated in 2015). Maybe CPython could incorporate a fork of pyreadline that fixes bugs (Unicode support in particular) and updates it to use a C extension instead of ctypes.