Thinking about this I disagree with some of the proposed categorisation issues.
The word 'fantasy' seems incorrect and I dislike 'crayonista' because it seems like a personal criticism of the proposer.
If having categories may be useful, perhaps "early idea", "developing idea", "ready to go idea"?
Playing the ball and not the man seems important here.
As to "fantasy", I would consider that to be something like:
"Avanti west coast should recruit elves for catering service, because they would be well suited the difficulties of serving food on a moving train"
This would be clearly be a fantasy idea. It is in every sense impossible, not least because I don't think elves would want to work in customer facing role these days.
However there are ideas that aren't currently feasible because there are some absolute obstacles but the idea still deserves serious discussion. These shouldn't be considered fantasy ideas, merely ideas with major technical obstacles or where the cost is too high.
To borrow an example from another industry:
Artificial satellites are an example. The first proposal was probably made by
Tsiolkovsky in 1903, and later discussed by
Potočnik in 1928.
At those dates the technology was clearly not up to launching such a device, if a suitable satellite could even be built at all.
But serious discussion was useful even if part of it was "it can't yet be done because we don't have a large enough rocket"
The first satellite then launched in 1957, 29 years after Potočnik's book.
I don't want to see discussion of an idea killed off just because there are blocks to implementation currently.
There might be a political or social change, some reduction in cost in the future or some technology might be created to overcome the obstacles.
Maybe with difficult ideas, we should discuss what needs to be done to overcome the obstacles?
As an additional point, there are ideas which are thought by critics to provide no value, however the question there is surely: "what is the proposer intending to achieve? Why do they feel there is value in the idea?"
Most of them tend to be that the proposer wants better service between certain points, so if the idea seems odd perhaps the discussion should be about how the achieve the original goals in a different way.