The two big things are a network stack that can act as a router (SeL4 has no network stack at all) and drivers. I;d imagine that a decent portion could be ripped from another open source operating system and run in userspace in SeL4 and get some benefits, but I'm not aware of any real effort to do that. Otherwise the usual way to use SeL4 as a hypervisor and just run linux under itm which gets you some benefits but probanly not the ones you'd want for a router to be as secure as possible.