1) Give you ability to "Uplift"(where you give animal men weapons/items/skills/armor or make them join your civilization) or exploit animal men tribes that aren't hostile. Also have they actually have name for their "civilization" and tribe leader and more details than being randomly found in caverns sitting around.
2) Make some gods to be fake. Or have world generation have "How much magic world has" choice with fewest having all gods to be fake and highest most of them to be real.
3) Random events like meteors, volcano eruptions, tsunamis, plague, earthquakes, year in which certain animal breed more, etc
4) Give underworld regions/caverns more details like names and stuff. Also return of tentactle demons(which could be just one kind of random element that demons can have) and demon kings(as in demons who rule other demons, like the "Underworld region" in the legends or the fortresses sealed with adamantine swords) would be nice.
5) Give it to some effect if you kill/capture megabeast that is worshipped as a god.
6) Place holder cities for goblins, elves and dwarfs until real ones have been greated. Its kind of boring in adventure mode espicially if humans are all dead.
7) Legendary carp/catfish/whatever living in the lake. You know those fishing stories about particularly huge fish that is hard to catch that lives in particular lake? Kinda like that, but an excuse to have a deadly carp again
8 ) Have there be astrology/astronomy, forgot the term, with each generated world having its own constallations, planets aligments and amount of moons.
Edit: Adding more suggestions.
(9 & 10 are in the next post)
11) Mines. Just so that human miners would have a reason to exist.
12) Have demons be their own civilization. Just so that when number 10 is done, they can enter at war with all civilizations and have epic war with them. There has to be consequences for the whole world if you release them
Speaking of which, hell having demon cities and not just forts would be cool... Of course, hell would be most hardest place to survive, but it would be interesting to explore hell as adventurer if it would have buildings and maybe other unique stuff.
13) Even if animal men won't be able to become civilization ever, at least have them be more tribal(at the moment, they just have blowpipes and spears and gather in group in underground with few pelts near them, have them have their own camp and maybe instruments or something) and maybe their own language which could be more understandable than kobold language while still being simples than main ones. Again, the coolness factor.
14) Be able to uplift kobolds. Why? Just because.
15) Have the "Give in to the night" option(the option you get when you get attacked by boogeyman if I recall right. Its basically same as "Give in to starvation") have the adventurer turn into a boogeyman/night creature.
16) Have night creatures(the random kind game currently has) be able to summon bogeymen at night. Mainly because that would give you a reason to avoid fighting them at night and bogeyman do share the trait of being randomly created
17) Use Alchemy skill and alchemy lab to transmute materials into another materials or creating hybrid/mix and match creatures like chimeras and stuff. That could be a way to make chimeras, centaurs, griffons, manticores and such. That or be able to create dwarf version of colossuses from different materials. Adamantine colossus which upon death creates masterpiece statue of dwarf anyone?
18) Have there be night creature which's M.O. is stalking, staring at you creepily distance ago and kill you when you are alone. Possibly also able to phase through walls. Anyway, I mean kind of Slenderman-ish monster which stalks you and stares at you before suddenly attacking.
http://www.bay12forums.com/smf/index.php?topic=100464.019) Have "Succumb to invasion" option transfer the fortress to the civ of invading force making you unable to reclaim it. Just so that you would have extra motivation to fight until all your dwarfs have been slaughtered
20) Have there be creatures who attack only during certain weather or cause certain weathers to happen when they appear. Extra points if you make it so that you can't see the creature that is hiding in the storm/sandstorm/blizzard before it attacks you, for example imagine this: There comes sand storm. You aren't thinking anything about it. Then suddenly, dwarf near your fortress entrance notices monster that attacks you and you are like "OH CRAP". While older players will instead know to be paranoid whether its just random sandstorm or the one that hides a monster...
21) Relating on previous one, have [insert biome] titans have some special abilities depending on where they are from.
22) If colossuses are ever made to be randomly generated, it could be done in this way: Have the material randomized and the which creature statue that you get upon killing them is. Like steel giant scorpion colossus which becomes masterpiece steel giant scorpion statue or bronze elf colossus that becomes masterpiece bronze elf statue.