I fully agree with JedMeister here. The way forward is not to attempt to merge appliances together but rather figure out how to easily run them as separate, self-contained units inside a virtualization environment. Merging multiple appliances together destroys much of the advantage of running an application as a virtual appliance to begin with.
I really like JedMeister's ideas for developing a "master" server that can easily run pre-bundled TurnKey Linux appliances. I believe there are several ways this could be implemented leveraging existing open source virtualization software such as Proxmox VE, oVirt, Enomaly, Ganeti, Eucalyptus and DTC-Xen. And I probably missed a few.
I think currently the main hurdle is for someone with a bit of time on their hands to experiment with the state of the art and figure out what works best for small-medium sized implementations.
Running combinations of virtual appliances
I really like JedMeister's ideas for developing a "master" server that can easily run pre-bundled TurnKey Linux appliances. I believe there are several ways this could be implemented leveraging existing open source virtualization software such as Proxmox VE, oVirt, Enomaly, Ganeti, Eucalyptus and DTC-Xen. And I probably missed a few.
I think currently the main hurdle is for someone with a bit of time on their hands to experiment with the state of the art and figure out what works best for small-medium sized implementations.