As of today, if there are no dwarf civs at all, and you've set it to allow no playable civs, it just won't list the "Dwarf Fortress" option in the title menu for that world. If all of the dwarf civs are dead, it'll still allow you to play, though I think it might force you to be the first one on the list or something silly the way it's set up. So, yeah, you can now play a world where the only allowable games are human "play now!" ones, or, if you mod it, you can make a world that only has a Legends mode, if you really want to.
Yeah, you can increase the volcanism field, which sets the frequency of magma features, and you can also set the desired number of visible volcanos.
The Caravan Arc is fighting for its spot vs Presentation stuff and Adventurer Skills I think. I'm sort of tired of doing just one arc at a time (I've enjoyed doing megabeasts and this parameters stuff more than the armies recently, just because I've been on world gen armies for a while), so the current leaning seems to be a mix of the three aforementioned arcs, though mixing in other stuff is fair game. We're going to talk about it a bit more once this release is out of the way.
A dark fortress with dwarves and gobs run by gobs has a population mix as the result of kidnapping. A dwarf fortress with dwarves and gobs run by dwarves has a population mix as a result of both prior kidnappings and dwarven immigration. Contrary to general player behavior, dwarves tend to be the most lenient with captured populations of any of the critters. The current human entity has slavers, the elves kill and (currently abstractly) eat their enemies, and goblins make grotesque body sculptures and so on.
If the dwarven leader of a goblin site is part of the goblin culture, they'll act like a goblin, otherwise the goblins will act like dwarves. It tracks all of their old affiliations, and the strengths of those affiliations, but having them act on those (to throw off the goblin culture, for example) is one of the items on the list I'll likely have to put off until next time.
There was a brief period of time in unreleased land last year where the migrant groups wandering the map could approach your fortress, even if they were elves or humans. Those have since been removed, but it'll be back sometime. Ideally, all migrants would come from actual places, though how this would work in certain worlds is a bit iffy. Perhaps it's not reasonable to expect migrants all the time, though they could always come from wherever your starting dwarves come from, unless those initial dwarves too are picked out from the population. Their skill sets would have to be handled then (and the point buying system might not be appropriate then, though it would be annoying not to have it). Still, it would be sort of cool to have a list of dwarves (including retired adventurers) in the current world that are willing to go off on a pioneer settlement journey and you can snatch up some of them and so on. This has been discussed in various forms and with various takes on how much of the initial journey should be abstracted. Anyway, stuff to look at.
Yeah, no humans in those frosty biomes -- you could always add additional human entities very simply by adding to the starting biome list (and we plan to do so in the stock raws later on). Right now, the human civilization is a sort of vague slaving plains-dwelling group with a loose site-based organization. Ideally, we wouldn't provide more stock raws rather than a framework to generate all the biomes needed of randomized human groups, but more stock groups will be a good stepping stone toward that.
Trees have always burned, though there might have been a hiccup in some of the releases where grass fires didn't burn hot enough for that to happen (this can happen in real-life, but a bug nonetheless). I think grass fires make trees burn now, but I'm not sure. Having a thousand degree environment makes trees burn, I'm pretty sure, though in that screenshot, I don't recall if there were trees or if the puffs of smoke were just the random ones that come off burning grass sometimes.
I can't say what happened at Site 76. Ask the government.
I think the phrase "sum of gravities" is misleading, although that might just be typical jargon for a wider class of algorithms. As dreiche2 mentioned, clearly something additional is needed if there are two equal positive attractors to your NW and NE, to keep you from just walking up to the midpoint between them and standing there, but interpreted loosely it makes sense. I can see how this could help you choose a basic direction to look at, for example, and you'd then think a little bit more about precisely what it is you want to do, though yeah, without specifics it isn't that useful. Perhaps they were in the White Paper -- I saw it before, I think, when somebody linked to Incursion, but the tone put me off.
Biome definitions could be moved into the raws, but as with spheres and other very basic concepts, it invites a lot of trouble, requires quite a bit of rewriting to the point of it just dumping C++ for scripts (though spheres are worse than biomes here, I think), and the gains vs. the work are pretty slight. If custom fields go in to the raws (the "evil" field is a good candidate for this), then custom biomes start to look more attractive. As it stands, there isn't much of a point, since the biomes are the biomes, as they correspond to the basic fields, more or less, though you could play around at the fringes of the acceptable values for temperature as we were discussing, I guess. I'm not sure what you had in mind.
The cave in the east surrounded by a swamp? I think that was just me meshing with the drainage brush and a lucky cave placement.
As for all the dwarven leaders in goblin civs in the frozen world, yeah, the kidnappers can be so effective that goblins essentially lock themselves out of their own civ. This'll probably be rebalanced sometime.
The editor just lets you change the six basic fields right now (elev, temp, rain, drainage, savagery, volcanism). Later, more -- doing midlevel maps and sites would be fine, but I'm nowhere near that yet... a site editor would probably work best after generation, since it needs a lot of the midlevel information ahead of time. Some of the similarities between individual cells have been decreased (the 'd' map looks less square now, for example), though there are still a few ramifications I have to deal with there, and it still looks pretty square if you make elevation differences too extreme.
It's true that the current world gen editors and so on don't have much of an impact on fortress mode at this point, but once we get around to sending armies from fortresses and conquering other sites from fortress mode and all of that, the particulars of the surrounding world will be crucial, and I'm sure there's a lot of fun to be had from specific maps as well as randomized ones there.
In general, world generation doesn't check up on a lot of modding issues and so on, so it won't look at missing lungs or anything like that. Their bodies are used during world gen combats, and their personality settings are used by their rulers. It calls things up as it needs them, and it assumes you haven't made basic errors, intentional or otherwise. In addition, aquatic races aren't really supported well at this time. I'm not sure what they'll do, as far as settlement and roads and all that, since the game makes some calculations according to land mass index.