Hi,
yesterday I stumbled upon Turris MOX, a modular computer.
It seems to be an interesting endeavor which might be easily extended with a graphic card module or any card module you could think of. The connection between modules is a standard PCIe connector (but no the PCIe pinout!) with pins of PCIe 1x, USB 2.0, and 2.5 Gbit ethernet.
https://www.indiegogo.com/projects/turris-mox-modular-open-source-router-wif...
Enjoy!
-- Jan
P.S. Disclaimer: I once met some of the authors in person.