Being able to create a new layer while other layers are running, then have the newly created layer depend on whatever layer's where running would be a very nice thing!
Imagine, I need to install an "external" compiler for MS Visual Studio 8. Currently, if I want to accomplish this, I must first install MS Visual Studio 8 not in a layer (I use VMware to I don't have to reinstall Windows when I'm done). I then install the external compiler (in this case the Intel C++/Fortran compilers) inside a layer, and name the layer to reflect the fact that it's IDE integration is tied into MS Visual Studio 8. Then when I'm ready to deploy, I create a layer with MS Visual Studio 8 in it and deploy to a box with nothing on it except SVS. I then can activate the VS8 layer and then the Intel C++/Fortran layer and it seems to work.
However, it'd be nice to have the Intel Compiler stuff automatically know that it's tied to VS8 and automatically enable it, or, not allow you to enable it unless VS8 is enabled.
In my scenario, I have 3 different VMware machines, one with VS6, one with VS7, and one with VS8. I then install the compiler on each VM inside a layer so I can get the proper IDE integration.
It'd be so much nicer to not have to rely on VMware to do this. This scenario could also be extended to things like an installation of Shockwave being tied to Firefox, or a Winamp plug-in requiring a Winamp layer.