The Spectrum Next seems to be the closest to the ideal, and obviously it's perfect for Spectrum fans including me, but it's missing:
• A built-in display.
• A platform not tied to imitating an older platform.
• A language not tied to an older language.
As for the others, despite being commendable efforts, they're also missing one or more of:
• Usable straight out of the box; no assembly required, and without need to connect and update the system first.
• A single official repository (not excluding others) for sharing and distributing user-made programs, accessible from the device itself as easily as the App Store.
• Standard keyboard AND standard game controller (most have only one or the other).
• No fragmentation: If you wrote a program for the Commodore 64 or ZX Spectrum, it would run on 99% of the C64s or Speccies (the only differences that I can recall were the 48K/128K versions, and things like "speech" packs etc. but we don't need separate "upgrades" like that today).
As "blondin" commented:
> i see people talking about the arduinos, raspberry pis, and all these micro devices for which you have to pick additional components among myriads and you will be very lucky if 10 more people have the same setup that you have. these people are missing an important point about platform and sharing here.
http://www.ic0nstrux.com/
https://www.hardkernel.com/shop/odroid-go/
https://www.hardkernel.com/shop/odroid-go-advance/
https://arduboy.com/
https://www.specnext.com/