i'm curious about this as well. Somethings I would like to see from Magento 3 is:
Strip out all mentions of themes and frontend from the stack as its obvious that Magento is prioritizing less of themes and addons in the admin panel as it's not easy to install or manage them anyways. Make Magento headless.
Push more for a leaner codebase and remove legacy code.
Remove any third-party code from the install and allow to install separately. This is done to allow for a leaner codebase.