I'm working on something like this, but as time has gone on I've made technical constraint less of a priority and focused more and more on the I/O and "bus" model itself, because that's where the ultimate bottlenecks lie. If you have all the processing power in the world but only a 256x256 space to draw in and a limited number of commands to draw with, you still have an interesting constraint.

One of the things I found myself doing with PICO-8's setup was prematurely optimizing code for size, which presented a new kind of puzzle(how to express things in fewer Lua tokens) but didn't feel good in terms of authoring finished work(I made a nice 3D maze wireframe renderer - and then stopped there). So with my own fantasy console I'm prioritizing other things beyond size/retro aesthetic, while still thinking about limitations. "Diversity" has become one of those things: you will have some ability to swap out graphics and sound cores, and use a variety of programming languages. The fixed spec approach makes it harder to make each part really distinctive and not just another "tiles and sprites and chiptunes" doddle.

Since I made the foolish decision to support 3D cores it's taken a while to get a renderer going, though, and I finally decided to speed things up by releasing with 2D first and adding more later.