The MNT Station, currently in development, could very well be a fully-OSHW "Steam Machine": https://www.crowdsupply.com/mnt-research/mnt-station

What about the graphics processing capability? I see there are some modules with a GPU on them. Are they good enough to run 3D games that were released in the last 5 years?

I'm also happy with older games. I just don't know how powerful these GPUs are.