A PnP placing the components upside down onto a surface printed by another head would be interesting. You could align the heights of the resting surfaces to optimise pads needing to be connected being on the same plane. I'd still want to lay copper but if you had the ability to squirt a little solder paste from (yet another) head, you could stack everything with wire connections into a very 3d circuit.
If the base material was thermally conductive you could have a heatsink block with the circuit embedded in it.
There's probably a sweet spot for material melting points for some printable substances. Hotter would be better if the printer can manage it. If you really wanted to make a solid block circuit inside a heatsink you could print it not worrying about things like layer adhesion, then once printed, place it inside a frame to hold it in place and reflow the entire thing. Would mostly depend on the ability of the embedded components to endure high temperatures during reflow, but considering how modern batch soldering works, I'm guessing a lot of this problems have been addressed (or a the the very least, the bounds of capability well known)
If the base material was thermally conductive you could have a heatsink block with the circuit embedded in it.