I made a custom tileset by Frankenstiening together a few others at first for the icons I liked most, then went and made several custom tiles.
I created custom castes for the "dwarves" that made them a more cosmopoliton civilization made of dwarves, minotaurs, sidhe, shapeshifting giant spiders, and lamia living together with a large number of changes in stats and personality quirks per race (spiders are superintelligent with ADHD, lamia are strong but tire quickly, and smart and artistically talented, but lazy), and made some custom tiles for that, too.
I made some custom reactions and crops, as well.
I was going to go making a more expansive mod that created its own mythos, more involved politics, plus things like laquerware and laquer bushes and such sort of ways to expand the industry of the game, sort of a Genesis Mod project, but got too involved in working out things on the Suggestions Forum, and got distracted.
Basically, I like DF more for what it can be made into than what it actually is, in some ways. I'm not really content with the sort of generic LotR fantasy story, but like mythology in general, so would prefer to be able to rig the game to be a bigger kitchen sink of fantasy components.