BlackMagic has worked with Apple to create an thunderbolt 3 eGPU with a Radeon Pro 580 in it. It is not upgradeable, but has an HDMI and four usb 3.1 connections and a thunberbolt passthrough as well as 85W of power. It is $699 and sells from Apple to work with their laptops.
Very cool, though I think they might have missed an opportunity in not combining it with a Decklink, Intensity or UltraStudio for playback to an external monitor. Even if it used 2 Thunderbolt connectors I think the addition of at least a playback card would have made this a must have solution.