There's two types of processes that I'm surprised I've not seen more focus on.
1) Printing of, and then filling of molds, which can then be melted down and reused. That's how the higher-end 3d printed parts that you can buy online made, including almost all 3d-printed metal parts you get from online 3d printing services (the extra steps for metal being to coat the mold in a ceramic shell and melt away the mold). The only commercial 3d-printed metal that I'm aware of that doesn't work in this manner is iMaterialize's titanium, which uses laser sintering - and it has an out-of-this-world price tag.
It seems to me that if you used a mold, while in several ways it complicates the process (extra steps, preventing adherence to the molded object, etc), in others, it simplifies it. Your print heads don't need to handle a variety of materials or produce a pretty or durable product. They still need to be able to produce fine surface details but the ability to print thin structures loses importance. Once you've got a mold, you open up the floodgates to the sort of products you can fill it with, anything that will harden either through cooling or via chemical reaction, anything from thermoset plastics to candy.
(note I'm not envisioning a little hobby home printer that fills molds with molten metal in your office, mind you... although I could envision a more garage-scale or small industrial scale version that could handle such a task)
2) I've never even heard of a 3d printer being based on thermal spraying. With thermal spraying, you can choose the balance of precision vs. flow rate via nozzle size. Your materials are virtually unlimited, pretty much anything you can turn into a powder. It could conceivably even let you work with metals in a home environment, if the rate was kept low enough that heat buildup wouldn't be a problem (and you'd want an air filter on the exhaust, even though it should be pretty clean). You can choose the temperature and velocity you're spraying at by varying the pressures of compressed air and combustible fuel fed into the chamber with the powder, so you can work both with heat-sensitive and heat-requisite materials, as well as materials that can't stand high velocity impacts and materials that require them. Such a system could likewise do more than just print - it could add and then sectively remove substrates, it could engrave, it could change surface textures by sandblasting/polishing with various materials, it could paint or apply high-performance coatings - pretty much anything you can envision from a device whose fundamental workings are "shoot grains of material of your choosing at a velocity of your choosing (1-1000+m/s) and temperature of your choosing (cold to thousands of degrees)".
In both cases #1 and #2, I'm genuinely curious as to why there's not been more work done with them. Or perhaps there has been work done with them that I'm unaware of? I'm asking as someone who makes and buys 3d printed items online but has never printed one herself.