Looking for custom game rule suggestions to improve challenge/gameplay

Coming back to the game after a year or so, I've realized that I've just been playing on default settings for 99% of the time thus far and I'd like to spice things up a bit. I start most games with a particular achievement in mind, but then role-play my way there (so rules should be achievement-friendly). Does anyone have any suggestions for game setup or self-imposed gameplay rules to increase difficulty and/or improve immersion? Also wondering if there are any achievement-friendly mods out there that achieve the same goals.