From what I've understood from various WoBs I've seen, before the lord ruler and lerasium, there were no mistborn, only mistings. And the further the bloodlines of those who consumed lerasium are diluted, the further the incidences of allomancy in the population reverts to its natural expression as mistings.
Ferrings by contrast, are not the natural form of feruchemy - they began to exist due to mixing of the Terris population with allomantic bloodlines. WoBs have talked about how the mixing of these two metallic bloodlines caused feruchemy to take on that 'only one power' quality of allomancy.
So while I think that in theory, through breeding, you could likely isolate the Terris bloodlines over time to see a return of full feruchemists, I dont think the same would be possible with allomancers without a new infusion of lerasium or something equivalent. I think at most you might see a higher proportion of mistings.
Also thanks for engaging with this all, there's been some good suggestions.