While it sounds great in theory, I don't think external graphics will ever be more than a very tiny niche product. The newest product in this category is the Devil Box from PowerColor, it supports both AMD and NVIDIA video cards and lets you link the card to your PC or laptop via a Thunderbolt 3 connection.
The Devil Box measures 400mm x 172mm x 242mm and fits a single video card with a maximum size of 310mm x 140mm x 50mm. It has an internal 500W power supply, has room for one 2.5" SATA 6Gbps HDD/SSD and is equipped with a total of four USB 3.0 ports and one USB 3.1 Type-C port.
PowerColor DEVIL BOX is based on AMD XConnect™ Technology which was introduced on March 16th, 2016 by AMD. AMD had developed this technology in collaboration with Intel’s Thunderbolt™ group and Razer Inc. The PowerColor DEVIL BOX is primarily for the users who would like to have an even more powerful graphics card on his/her notebook in pursuit of desktop-class graphics performance. With a qualified graphics card, not limited to either AMD or nVidia, installed in the PowerColor DEVIL BOX and connecting with Thunderbolt™ 3 specification, gamers and enthusiasts will be able to play any types of games on the market without sacrificing the portability of a thin’n’light notebook.
The big problem of course is that just the box alone costs almost as a high-end video card. PowerColor says it will be available in very limited quantity for $379 (419EUR) from places like Newegg.com, Mindfactory.de and Overclockers.co.uk.