Dangit, I had a long post put together, but I just lost the whole thing. Here goes again:
Lofty ideals for DF Modding, all possible (though hardly expected):
1) The creation of varied entity races is neat for generating unique and strange worlds, and it's neat in its own right. However, just diversifying the existing entities with variants that look superficially the same (the game still calls them humans, dwarves, etc.), but which behave different mechanically does the same job, and stays more true to DF... while also making it so that you never know what to expect from your neighboring civs, and always generate unique-feeling worlds. I wouldn't be averse to a few things added in that stay true to established mythology and fantasy, but the existing DF Civilizations have a good dynamic going on. It makes it so that the player really doesn't know what to expect from any given entity, and allows two entities of the same race to trade, wage war, etc. In any case, entities ought to make use of the abundance of different biomes in the game. Civilizations can be found the world over, from the hotest and most hostile deserts to the densest mountain jungles; the aboriginal Canadians survived and thrived beyond the treeless reaches of the arctic circle, using bone for tools, hide and ice for walls, and whale fat for fires. Though all need not be playable in fortress mode, having custom reactions and entity positions for those which are, allowing for balanced, challenging, and thematically sensible gameplay would be nice. With a large pool of possible variations, you'd end up with different civilization interactions every time you generated a world.
Hypothetical "Entity Flavor" examples:
a. Shamanistic desert elves, who throw bolas, enslave their enemies, ride wicked scorpions, and envenom their weapons
b. Troglodytic dwarves who siege from the Depths, babble nonsense, and wield spiked fungiwood clubs.
c. Coast-dwelling amphibian men, which "grow" Muck Oysters and harvest them for mid-value pearls, which you may find encrusted on their tridents, as they ambush from your aqueducts.
2) Expanded industries to cover basic technologies, and for non-dwarf civilizations; things like neolithic toolmaking, coastal industries, alchemy, etc. Hewn Quartzite anvil-stones, carved antler pickaxes, bog-iron, knapped stone weapons, etc. Primitive or hunter-gatherer civs oriented toward animal products may use barred bone mail, shell/scale plate, etc. (even more interesting if certain creatures have exceptionally strong or valuable bones/shells), hide beds, and general bone/horn/stone tools. Also, for surface-dwellers not adept at fine Stonecrafting, creating a ceramics industry which could produce clay or porcelain goods at a kiln would be nice. Poisoning of weapons to whatever degree is possible would also be nice (passing fancy - can objects made of or decorated with a syndrome-causing material cause syndromes on a strike? If so, poisoned weapons, or [hypothetically] "enchanted" weapons are possible).
3) Inclusion of more creatures, particularly ones which are notably unique or iconic. Having 17 species of Gibbons or 23 species of Turtle might be realistic, but if they don't each bring something new and interesting to the game they're largely redundant. However, adding in the octopus, squid (with a giant varient), and cuttlefish, or even just an octopus, would bring new and iconic things to the world, which it lacks, and in the event that your adventurer is crossing a narrow ocean bay and winds up engaged in a grappling match with an octopus, or your trapper manages to put a tame one into an aquarium for a noble with specific tastes, it makes it all the more cool. Large predators like giant snapping turtles, wendigo, or even giant bombadier beetles capable of spraying severe blister-causing liquid at enemies would add variety to wilderness travels. As for megabeasts, throwing in more mythological monsters like Landwyrms, Rocs, and Treants would be nice to see. Even creating an ephermeral megabeast that serves as an Avatar of the elven Forces, with the [MAGICAL] tag, such that when it sieges it brings a horde of angry wildlife with it, could be quite cool.
In summary, what I want in a mod is a set of gameplay elements that work well together, build on what is cannon in DF with new industries and options, providing ethical/technological variations on the existing base civilizations such that your world generates with a different flavor each time (with additions staying as cannon to DF and existing mythology as possible) each with different technological and social structures in place, all wrapped in a shell of solidly interacting, well-built fantasy. A lofty ideal, but there you go.