So Robert Yang posted a decent guide on how to ‘sub-project’ your Unity projects so they can share a common code-base easily. This does what it needs to do pretty well, you can easily add to and maintain your own framework, and build individual games on top of it. This, as he warns, comes with some complications. You have to juggle different build/player/editor/project settings, each game’s /Resources folder will be included with every build, and you need to manage an ever-growing script base. I manage my shared code a different way, though the magic of Git submodules.