btw I think MESS will at least need some pre-canned 'installed' versions of things like the OS as recognized software images which can be specified / mounted. Having to install a game is one thing, having to mess around and install entire OS each time you want to try something new, especially under an emulator isn't really productive or useful.
I agree, at least some plain installs for significant versions of OSes, but trying to keep it at the minimum. I would say the minimum amount of pre-canned OS images so that all the software lists for a certain system can be run, preferably only one. Also things like a DOS boot disk for PC and things like that.
The problem I see here is, what size CHD do you create for each of the pre-canned OS installs?, do these files grow as the space of the CHD get's filled up or they are a certain size from the start?
I think this has to be handled with a lot of care, as already was said, software lists for certain systems can be almost infinite. In that case, I think only some minimum pre-canned software should be included and that's all.