As far as I see, humans need plains or possibly hills and it can't be evil or savage. They also turn forests into those plains or hills during world gen, but I'm not sure how they go about that. It may be that they need to conquer a forest retreat, or they may build their own as they expand. If you have mostly deserts, tundra, swamps or mountains with high savagery and evil, then humans won't have anywhere to live.
Thanks!
I think I've figured it out -- I just had to turn my Savagery min down a bit, lower the min elevation to 190 (to get some plains), and reduce my evil square count slightly. The following parameters seem to generate a site with magma, flux, HFS, a surface brook, access to all four civilizations, and high elevation counts pretty much every go, and sand about half the time. You can usually find a few suitable sites by having the finder search for other factors you don't necessarilly need, like a chasm etc.
[WORLD_GEN]
[TITLE:WOOKIN]
[DIM:257:257]
[END_YEAR:1050]
[BEAST_END_YEAR:2:50]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:190:400:1600:1600]
[RAINFALL:25:100:1600:1600]
[TEMPERATURE:25:75:1600:1600]
[DRAINAGE:75:100:1600:1600]
[VOLCANISM:50:100:1600:1600]
[SAVAGERY:50:75:1600:1600]
[ELEVATION_FREQUENCY:2:10000:1:9999:1:10000]
[RAIN_FREQUENCY:2:10000:1:1:1:10000]
[DRAINAGE_FREQUENCY:2:1:1:1:1:10000]
[TEMPERATURE_FREQUENCY:2:1:1:10000:5000:10000]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:2:10000:1:10000:1:10000]
[GOOD_SQ_COUNTS:2000:4000:15000]
[EVIL_SQ_COUNTS:2000:4000:15000]
[PEAK_NUMBER_MIN:25]
[OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:200]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:0]
[RIVER_MINS:800:800]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVE_MIN_SIZE:500]
[CAVE_MAX_SIZE:500]
[MOUNTAIN_CAVE_MIN:800]
[NON_MOUNTAIN_CAVE_MIN:100]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:100]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
cant i turn off one of the erosion settings?
Yeah, sortof. If you turn off all erosion, you'll still get near-sheer cliffs, but they'll have a slight slope to them -- one or two tiles of ground or ramp per z-level. You can dig that away fairly easily to create a sheer cliff, though.
Other option is look for sites where a brook cuts through a mountain.