Great idea! Admin notices are very over-used and really take over the WordPress admin. Theme and plugin authors think their theme or plugin is the most important thing in your life, but they're not in a position to judge. Even a seemingly important notice, for example one that appears when you activate the theme or plugin, can end up being irrelevant - e.g. it may have been intended for people to see when they first activate the theme or plugin, but end up annoying people who are just temporarily disabling things for testing purposes. A central notifications center would even out the playing field and stop plugin authors from competing to have the most prominent notice. It would bring the admin back to what it should be - a place to edit and maintain your WordPress website, not somewhere to be distracted with unimportant clutter.