Here’s a couple of threads that might be interesting for you to read through, including my collection thread.
https://www.elitefourum.com/t/attempting-to-categorize-vintage-japanese-promos/
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I think the longterm “viability” of Japanese promos is a little enigmatic. From the perspective of story and population, Japanese promos are often some of the rarest and most interesting cards released to the public. But they also ask more from a collector. Unless you are Japanese, you have no personal connection to them or even an awareness of their existence.
So I think the future of Japanese cards might honestly be in the hands of, dare I even say it, influencers.
Edit: In western countries, I mean.