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A Fantasy Console (FC) must not have existing hardware.
E.g. The arduboy is not a fantasy console.
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A FC needs to have intentional restrictions not related to the hardware it is being deployed on.
I feel adding the retro component (e.g. looks like X) to be defeating, as it might stifle the creation of new fantasy consoles. At the end of the day, a FC is based on a fantasy hardware product that is difficult to alter once released. For example; the fantasy console may only have a few MB of memory, whereas the system it is being deployed on may have GB of memory.
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A FC may be influenced by retro hardware.
While most FC's already are influenced by hardware choices of the past, I don't think it's a hard requirement. I feel that while the nostalga factor is awesome, but it can't be required.
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A FC must have tools or documentation included.
I feel that requiring built-in tools being necessary is also beyond the scope of a fantasy console. Old NES hackers remember having to make their own software and hardware just to dev for it. Requiring tools is a very nice thing, but it can't be a requirement. Regardless, documentation or tools should be included, as you could not find a console without one or the other.
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It must have some form of input.
Buttons, keys, controller, etc. The user of the hardware has to provide some form of input
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It must have some form of output.
Lights, screen, speaker, etc. The user of the hardware has to be provided some form of output. You could make a controller console that only outputs sound. That would be a legit, albeit strange, FC. RAM can't be a requirement - I imagine there could be some interesting FC's that only have CPU registers (e.g. the 6502 only with registers). ROM is not a requirement (e.g. the Altair 8800).
This is my (@josefnpat's) opinion, and is more than open to debate and discussion.