In user story format: As a module developer, I want to define my config in a PHP array (which I know how to write), so that I don't have to fiddle with XML or YAML or whatever.
I don't know exactly how this would work, but I would expect it to take an associative array as input, and output a config file (as a textarea or a downloadable file or whatever). Whatever we end up with, we also need a drush task for it. :)
I know this is the kind of tool we have traditionally stuck in devel, not core. I'm not convinced this belongs in core, but if it goes in devel the people who need it most may not find out about it. Maybe it should be a tool on drupal.org instead. I'm filing it against core so we can think through the best solution, and then we can move the issue wherever it belongs.