Dig beneath your planets. Freight Elevators sink shafts to caves three levels deep under Nauvis (four with Space Age, plus one under Gleba and one under Fulgora, and 2 under Aquilo!) The ore above runs on below; deeper down come granite, quartz, feldspar, geothermal heat, and higher pressure, guarded by Underground Horrors. New buildings, new enemies/combat, new defenses! Supplementary to Space Age, no required technologies but lots of less expensive alternatives. Build deep, defend, expand!
Large total conversion mods.