With the coastline, mountains, and rivers placed, we’re set to drop in everything else we want. And you probably want most other things. Granted, on an island this size, it’s probably not reasonable to include both arctic ice-sheets and tropical jungles. Still, snow-capped mountains surrounded by tropical jungle is certainly feasible, as are the sorts of rain forests you find on the coast of the Pacific northwest creeping up towards tundra. If you really want it, you can probably find a way to make it happen.
Why go to such extremes? It’s not really necessary, but you may want to be able to drop in things you pick up. Death Frost Doom kinda requires a frozen landscape to work in, though a good, tall mountain might work. N1: Against the Cult of the Reptile God works best if you have a swamp. B4: The Lost City could be dropped in a jungle, but the feel is decidedly desert.
Besides, endless days of nothing but forest get dull. You want to keep things fresh and interesting. Varied terrain also makes picking a route more interesting. Traveling down the river is faster than cutting through the hills, but if you do that, you might miss something interesting. And maybe that river doesn’t go where you want to go.
Before we start plopping terrain, a word about climate. Right now, in the tropics, the prevailing winds, and the rains they push, move from east to west, and in the temperate regions they go the other way. This is what creates all the moisture in the Amazon as the rains that don’t fall over the jungle are forced to dump their moisture when they run into the Andes, while the Pacific coast of California is rich and fertile while the other side of the mountains is a desert.
But the world was not always as it is today. The Sahara was once a lush forest. Ferns once grew in the soils of Antarctica. If climate’s a useful tool for you, use it. Some people feel climate is a good framework to use when the blank hexes grow too daunting. Otherwise, feel free to chunk it.
Could you have marsh next to desert? Sure. It’s easy to imagine something like the salt marshes outside Lankhmar along the edge of a desert, or maybe even as the dividing line between desert and sea.
Since I’ve said my island is tropical, I’m going to put lots of thick jungle on the eastern half. I’m declaring the western half is mostly savanna. Putting the PCs’ base near the dividing line in the north gives them an interesting choice almost immediately: dense rain forest or open grasslands?
But this is a fantasy world, not just Hawaii writ large. Normal terrain is good and all, but if you can, you should absolutely drop in some really cool fantastical bits of terrain: rivers of lava, rocks that float in mid-air, columns of stone that have been hollowed out to be the homes of gnomes, landforms that are actually giant creatures, mammoths’ graveyards, magically warped terrains, groves of treants and dryads, burning hellscapes, great plains teeming with carnivorous plants, yellow brick roads, rainbow bridges, gumdrop mountains, towers that shift between different dimensions, tesseracts, enchanted forests, eternal storms, ancient standing stones, radioactive wastelands, crops of magnetic crystals, and other cool ideas I’ve never even dreamed of yet, but that I hope you’ll share with me.
I'm not touching the spot in the middle, our mysterious plateau, just yet. Still not sure what I want to put there, but there's no rush on that score. As rorschachhamster pointed out on Monday, that lake really is just crying out for an island in the middle, isn't it? But first, I think I want to decide what my major populations on this island are going to be. We'll tackle that on Friday, if all goes well.