Many thanks for all the insights.
I understand that the mask cost will take a big share on the project. But what you call the soft core must carry a certain work, time and cost too. The idea is to make it public all the design processes and optimizations tasks, that are not part of the mask manufacturing itself. We will need to subcontract an "open" fab or fabless provider with no NDA issues.
Does it make any sense ?
Yes, making a design isnt free, but for something as "simple" as a bitcoin asic, thats almost noise compared to the cost of a maskset on a modern process. The actual cost isnt even so much the time of the engineers, its the software (and hardware) they use to produce the design thats ridiculously expensive. Who is going to contribute that?
Also if you factor in the disadvantages of a softcore, its possibly not even cost effective to use it, even if it was available for free. I could be wrong there, but it wouldnt surprise me.
If you want to do something opensource, I would encourage you to look in to s-asics, particularly hardcopy by altera. Those are essentially hardcoded FPGA's. Compared to regular FPGAs, they appear to offer significant advantages on almost every important metric: die size, power consumption, and performance. Compared to ASICs they offer a much lower NRE.
Alll you need for this to work, is a proper bitstream for Altera FPGAs. Once you have that, anyone can go to altera with that bitstream and order a batch of 50 or 100k s-asics for a price thats at least more manageable than a true asic. What I dont know is what kind of performance you could eek out of an altera fpga.