As part of the Armies Arc, civs need to be able to build roads. This will be especially important once armies are in because while an adventurer can travel through thick jungle at a regular speed, to an army that terrain would be practically impassable. The same is true of anything larger than a brook, swamps, or even a few miles of soft, sandy desert.
While Dwarf mode armies (and trade caravans, and migrant parties) would be the main beneficiaries of roads, Adventurers moving along a road could at very least have a reduced chance of being randomly attacked... by animals. That is assuming, of course, that that civ is wealthy and strong enough to defend it's roads.
Other benefits of road systems include that it provides another way for the player to interact with the game. I do not know how the player would direct the building of roads, but please add a way to do so other than normal Fortress mode, otherwise players would end up with worlds criss-crossed with abandoned 'fortresses', and heroes would run into parties of feral dwarves every few miles along the path.
A final note, for civ-built and player-built roads, there should be some automatic traffic designation so that NPCs feel like walking on the nice, smooth road.
'I'm PTTG?? and I support this suggestion'