First, I'm using accelerated DF for stuff like one kind of wood, one kind of leather, one kind of meat, etc for ease of use.
I also lowered the size requirement of mauls and halberds so that dwarves can wield them, and made them native to dwarven civs. Other weapon/armor modifications include adding chainmail hoods for extra defense, and making "war cleavers" and "war carving knives" which are basically copies of the tools that are supposed to be so OP in adventure mode. This way they are classed as weapons and can be forged and assigned to military in fort mode.
I added custom goblinite and hemagoblinite ore to be mined in game that has a chance of producing copper, silver, bronze, or iron, or a chance of iron or steel, respectively.
Other stuff include some underground variety such as giant cave crocs and blind cave badgers, as well as a giant variety of the blind cave badger. Also thinking of making an extra size tier of underground beasties that are twice the size of giants and calling them titanic -insert critter here-.
I'm thinking of also making all stone --except flux and stuff like microline, sunstone, obsidian, or cinnabar--a uniform gray color. I might do the same with minerals, actually, so that it becomes less of an annoyance when my dwarves discover a vein of hematite running through the middle of the carefully planned dining hall.