The decoupled Symfony components have been a great help but there are a few things in FrameworkBundle that could be moved to the components with some small refactoring making them more useable for people like me who have to use the components without the full framework.
There's a pretty good chance I'm overlooking something with some of these and there are certainly backwards compatibility concerns to be adressed.
Let's dig in:
The warmUp() method from the Router class could be dropped (also the WarmableInterface), the code would move RouterCacheWarmer. With this change RouterCacheWarmer only depends on HttpKernel and Routing and could go into either of these components. (My favorite would be Routing)