pouët.net

That is not fantasy. What you are describing is exactly the Dingoo, for which a number of demos have been made.

Make a demo for each of them instead!

In that case, buy an used console or computer and have fun :) (I do that as well).

I know the bitbox quite well, it's just a home-made thing (just a PCB with a chip on it and a few connectors). The original creator did it for learning electronics and low level programming, I think in the long term such knowledge can help people become more aware of electro-junk (you won't throw it away if you went through the pain of soldering and assembling it yourself, I guess).

Also, there is a "simulator" if you don't want to own the hardware. It generates Linux (and possibly Windows but I didn't try) versions of the programs.

Or the Game Boy Advance. Or the GP32.

that's not exactly what I had in mind, the GBA has a lfb mode, but very slow and just meant for pictures, but lots of fancy modes with sprites and so on, so demo prods will surely use these capabilities. Just fast direct graphics memory access (compare DOS VGA Mode 13h or windows lfb library pixeltoaster) would be interesting for competitions, since everything has to be coded from scratch without hardware support ;)

GBA might be kinda slow (but enough for my taste) but lfb (if you mean the chunky 16bit lowres modes) was used more than for pictures as far as I remember I have seen many smooth enough effect pure software rendering on the lfb buffers.

One good thing pico-8 has is an online showcase outside of the demoscene. There's a few demos sitting in their top rated chart among the games and tools.