I'll definitely be trying the copper once I get my 3D printer up and running, but being an aerospace guy, what I'd really like is aluminum for its light weight. Preferably one of the structural alloys like or 6061 or 7075. Another potential candidate is the AlSi10Mg alloy that 3DSystems uses for direct metal printing (looks like it's also called A03600). But even pure aluminum would be good.
Probably will take some experimentation to optimize, though. It will likely have a lot of oxide mixed in by the time it makes it out of the kiln, and combined with the porosity left by the PLA binder, who knows what the exact properties will be.