- I'll chime in with my two cents:
--- The ruins idea could be a facility which generates new TN Minerals on site, with the caveat that you still need to mine it yourself. The ruins could have a means of expanding their production, via Xenoarchaeology. Similar in the vein of the old Xeno Teams from VB6, you'd have a chance to discover a possible "expansion", but you'd need to send in Construction units to actually bring it online. I think they should be fairly common in ruins, but with the caveat that they produce specific minerals, not the full package. Maybe multi mineral ones and a full package one could exist, but be much rarer. Then the chance of discovering an expansion would roll for either more of the existing mineral produced, a higher accessibility, or a new mineral altogether. his could tie in with a ruins-only tech that would lead to being able to build your own facilities.
--- The Nebulae could have it so that they roll for minerals like a gas giant, with the nebulae level determining how much it produces per annum and the accessibility of it. You could then have ships use Sorium Harvesters to gather it up. This would provide a small trickle of infinite fuel, unless you just so happened to have a HUGE nebula; however, such a nebula would become a highly contested resource and you'd need to protect it. Thus this version of the Nebula resources would facilitate a more widespread use of Electronic Hardening and specialized HPM ships for defense and raiding of enemy Nebula mining. I'd suggest making the per annum similar to the current Starting Mineral Generation, with every level of Nebula counting as 10%, while the accessibility should be completely random... or at least the same means of determination used for everything else.
--- The Black Hole on the other hand should generate using a boosted version of the Homeworld Mineral Generation routine. Then that result should be reduced by a percentage based on the Class of the Black Hole, with a Class X having no reduction and a Class I being only 10% of the result. The accessibility should likewise be determined by the Class of the Black Hole, with a Class X having 1.0 and a Class I having 0.1, this makes the more powerful ones more lucrative than the weaker ones. Such you would a powerful vessel to do any meaningful harvesting, and as you may well need protection for the better finds, you would again need special ship designs to defend them. You couldn't use Jump Point Stations either, since they would get sucked in, thus Jump Point Assaults become a significantly higher threat to any such operation.
--- Finally, I think all three of these options should be locked behind Ruins-Only technology. The facilities require tech to develop and build. The Black Holes would require a special mining module to harvest, while all three would require special Geological Sensors to do a survey for those regenerating minerals in the first place. This new sensor could also have the benefit of functioning as both a Geological Survey Sensor and a Gravitational Survey Sensor for the purposes of conducting regular surveys. Along with Improved, Phased and so on improvements. The new sensors could be more expensive and slightly bigger to compensate, like 300 Tons and twice as expensive as a standard version of either. Ground Team Surveys would NOT improve these prospects on planets, but the enhanced survey would make them re-roll for eligibility.