TLDR; I've created a World Gen parameter that creates large islands, which, increase in difficulty as you move towards the center of the island.
So...
I've been messing around under advanced world generation more than I have actually been playing. Whilst trying to create unique worlds I'd actually like to play on I've learned quite a bit trying to make world gen ideas come into a playable form. Today I made an interesting discovery I thought would be worth sharing.
I started trying to make large worlds with minimal ocean and lake areas to maximize playable area. I also was cranking up dragons frequencies, pop ratios, cluster size, etc. and reducing the age for dragons to reach full size to 400 years. This way reducing the age they reach full size isn't so cheaty, but, allows for reasonable world gen times since most dragons spawned at world gen are any where from 150-300 years old. This is all to make Dragons the more dominant and prevalent mega beast life form. I found it difficult to find a good civ count, in combination with site and pop. caps, that wouldn't give me civs that sprawled the whole map or were so few in number and sites the map was unusable. I was using a high degree of variability in terrain to create numerous amounts of individual sub regions. Again running into issues with where civs were being placed and how their pops were generated.
So, I instead began making Large Islands instead to push civs closer, but, not to close, not to large in population and, had more sites with smaller pop. numbers instead of one or two sites with 10,000 plus pop. (creates huge lag in adventure mode). I took what I learned from trying to create vast almost water less worlds to maximize playable area, terrain variability, civ placements, beast counts, etc. and applied it.
Starting with typical island formation parameters and ending with a high variability for randomly generated in game things like curse and what not. I defined a large mountain cave number to allow for the amount of beast I was looking for and removed non mountain caves entirely. I also defined less terrain variability to give me mountains which are usually centrally generated in the middle of the islands. This combination in particular, places the deadlier cave dwelling beast around and in mountainous regions only, along with a large semi megabeast number, to allow for less deadly creatures to inhabit the rest of the world, is what allows for this tiered difficulty to happen.
Often I'll get unique terrain features such as oasis style biomes in the center of the mountains and valleys which you can see in the screen shots included below. The lair concentration which contain the more deadly beast are much higher in concentration around the mountain areas. You can also see that my mountain and cave dwelling civs (dwarves and kobolds) have been, essentially, wiped from the world usually due to large beast attacks. The remaining civs and civ sites are ones that have maintained a position around the perimeter of the island. This tiered difficulty concept could be useful to many RPG experiences depending on how you may want to play.
Summary: Civ and site placement overview: World Gen Parameters: [WORLD_GEN]
[TITLE:LARGE ISLAND]
[NAME_SEED:wwoWIiMEwcicc6EuqIsS]
[DIM:257:257]
[EMBARK_POINTS:1504]
[END_YEAR:169]
[BEAST_END_YEAR:2:66]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:400:400]
[RAINFALL:0:100:400:400]
[TEMPERATURE:25:75:400:400]
[DRAINAGE:0:100:400:400]
[VOLCANISM:0:100:400:400]
[SAVAGERY:0:100:400:400]
[ELEVATION_FREQUENCY:1:3:2:0:0:4]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[POLE:NORTH_AND_OR_SOUTH]
[MINERAL_SCARCITY:4000]
[MEGABEAST_CAP:150]
[SEMIMEGABEAST_CAP:1000]
[TITAN_NUMBER:5]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:69]
[NIGHT_TROLL_NUMBER:69]
[BOGEYMAN_NUMBER:69]
[VAMPIRE_NUMBER:69]
[WEREBEAST_NUMBER:69]
[SECRET_NUMBER:69]
[REGIONAL_INTERACTION_NUMBER:69]
[DISTURBANCE_INTERACTION_NUMBER:69]
[EVIL_CLOUD_NUMBER:69]
[EVIL_RAIN_NUMBER:69]
[GENERATE_DIVINE_MATERIALS:1]
[GOOD_SQ_COUNTS:24:244:0]
[EVIL_SQ_COUNTS:24:400:0]
[PEAK_NUMBER_MIN:12]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:4]
[VOLCANO_MIN:4]
[REGION_COUNTS:SWAMP:252:0:0]
[REGION_COUNTS:DESERT:252:0:0]
[REGION_COUNTS:FOREST:1500:0:0]
[REGION_COUNTS:MOUNTAINS:3000:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:2016:0:0]
[REGION_COUNTS:HILLS:2016:0:0]
[EROSION_CYCLE_COUNT:350]
[RIVER_MINS:50:50]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:75]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:25]
[CAVERN_LAYER_WATER_MIN:20]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:15]
[LEVELS_ABOVE_LAYER_2:10]
[LEVELS_ABOVE_LAYER_3:15]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:20]
[CAVE_MAX_SIZE:40]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:0]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:40]
[TOTAL_CIV_POPULATION:10000]
[SITE_CAP:1200]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:33000:4032:2016]
[RAIN_RANGES:2016:4032:2016]
[DRAINAGE_RANGES:2016:4032:2016]
[SAVAGERY_RANGES:2016:4032:2016]
[VOLCANISM_RANGES:2016:4032:2016]