Right now, AFAIK, every evil area has foul fog, evil rain, AND undead. When do you plan to give evil areas more variance/personality? For the most part, you've already explained your vision for non-binary good/evil sphere aligned areas, corruption by historical events and malevolent intelligences, and cleansing/purification, so mostly I'm just asking when and how? Like, what sort of stuff would go along with this or happen at the same time, and what needs to be in the game before you can even start working on this?
Generally, questions of when get the answer "No timeline, but it's planned", and questions of how get the answer "We haven't worked out the details yet." As you quoted yourself, the eventual plan is to scrap the Good/Evil regions in favor of sphere-influenced lands, and Toady has mentioned that not all spheres would be suitable to become land-influencing. I'd actually say that many of the moving parts required are already in place. We have randomly-generated creatures, we have interactions, we have sphere links on materials (though technically they wouldn't be placed yet), we have the ability to generate things after history starts running (because, let's face it, it's always !!sad!! when DF generates stuff that can never see the light of day - many of the generated evil weathers, and now spoiler materials, and depending on your settings, unique demons won't be used anywhere). There's still several moving parts that are needed to make those regions interesting, I'd say, as well as lots of detail work, but I'd say Toady might be able to add it in now.