With the patch we have seen all mines become perfectly equal, with ore quality increasing with distance from the ladder. While this has eliminated the NEED to scour the earth for high purity nodes, it has I think gone a bit overboard in eliminating both this activity AT ALL and has robbed some folks of their invested effort in thouse searches.
I propose that we go to a kind of hybrid system, purity increased with distance from the ladder, but purity AT the ladder starts at the mines old pre-patch quality level and grows as normal. In essence the old high purity mines just get you a jump on digging the long ass tunnel that were all going to be working on now. The old purity levels should still be retrievable as they were certainly generated with some kind of perlin noise algorithm which can be re-run to patch in the old values.
While were at it another factor could come into play, a bit more of a 'head wind' when it comes to mining that long tunnel. I'd change several things, first the cave-in danger should increase with distance from the ladder (meaning that higher M&M is needed to balance it), second all mine tiles have a soak value which ingresses likewise, not something that would require a Titan to mine but enough that a miner would need to be diligent in raising phlegm to continue the advancement towards higher purity. In H&H mining was considerably more 'physical', it was a large food drain and it took some burly characters to do it, and people liked being Miners, it was a recognized specialty, even a profession I'd say, one Salem has not really developed to the same degree.
Lastly I would make a large portion of the underground tiles non-ore (Gangue) tiles naturally, their should be a clear visual difference between the two, the gangue yielding just like what we get from cave-in's currently (worthless rock). The ore will be in veins (generated via perlin noise) that the miners would needs to intentionally follow, the pattern of ore quality improvement might also be made non-uniform, for example being more lobe shaped rather then circular so an unplanned strait line tunnel while it will eventually find better ore would not be the optimum strategy. Of course people can 'go for broke' and just mine strait exploratory tunnels if they wish, as with everything Salem you take the risk, we harvests your tears.