I have a project I worked on with the RPi3 called "CocoPi3." It's based on Raspbian and focuses on TRS-80 Color Computer / MC-10 and Dragon emulation. It does NOT use RetroPie as a base. I compile the latest version of MAME (0.189 at the time of this message) as a console application (without the need for X windows) so speed is real good. I also offer XRoar as another emulator for Coco 2 and Dragon emulation. In addition, I added a ton of development utilities and a menu system. There's quite a features packed into it. There are a few places to download it and the documentation:
I have moved the project a bit further and installed my RPi3 into an actual Coco 3 case and make use of a Coco 3 keyboard to USB adapter (via the use of a Teensy 2.0) controller. Chris Hawks was the original person who did the work and I leveraged it for my project.
In any case, I'm a fan of the TI-99/4a and the Atari 800XL, so I'm thinking of adding support for those 2 platforms as well. This would need to include development tools, etc..
Here's a link to some pictures of the RPi3 (with the CocoPi3 distribution) in a Coco 3 case running Coco 3, TI-99/4a and Atari 800XL emulation.