It would be nice if we could have wildcard slots in ATOMs. Let's say an user wants to only long term stable kernels, then we could put SLOT="${PV}lts" in those ebuilds and allow users to specify :*lts in the world file. Or if wildcards can only be put at the end, we could make it like SLOT="lts-${PV}" and allow users to specify :lts-* in the world file. This feature would be helpful for bug #497994. Maybe this is also applicable in the Portage tree, *unsure*...
Thinking further, this also makes it possible for users to depend on kernels in a certain branch; for example, specifying :3.12* in the world file would get kernels with SLOT="3.12", SLOT="3.12.1", SLOT="3.12.2", ...
Sounds like a new can of worms. Remember that we need to distinguish 'same slot deps' (:=) and 'any slot deps' (:*).