I've been steadily refining an "ideal" worldgen for myself in preparation of the upcoming release. Naturally, since worldgen situations will vary greatly as a result of the artifact release, this has been focused mainly on geography. However, more broad civilization tweaking has also taken effect. Manipulating the size of available settlement regions and cranking up the good/evil biome numbers, as well as greatly amplifying the default savagery levels then tracing out "routes" of reduced savagery, has led to much more controllable civilizations.
Of particular note, the most influential factor for coaxing towers reliably out of a world (some as early as year 17!) was simply adding the [DEATH] sphere to the dwarven pantheon in their entity file. Considering the game motto is Losing is Fun, in retrospect it almost seems bizarre that dwarves don't have a god of death by default. I know I certainly hope for a suitable death for my forts, and regularly conduct rituals to avert Death by FPS. With this in mind, I'm going to continue to tinker with other entity-level modifications to influence worldgen results. A drawback to this, however, is the usual "necromancers write a thousand books every year" clutter.
Meanwhile, to deter the usual total goblin takeover of the world while still keeping them an active, threatening force, I instituted an actual maximum age and sharply cut into their population/site cap numbers.
Otherwise, some modding was done for the sake of framerate. For example, I adhere to a single cavern layer, because maps including more than one quickly slow down into the dreaded 10-25 FPS range even with a strict population cap of 40. So I adjusted the underground flora and fauna to spawn on the first cavern layer. Meanwhile, trees causing pathfinding issues was something I've tried to counteract by reducing the heavy branches and doubling average tree height, thus making it more difficult for dwarves to end up in a "I've gotten up and can't fall!" LifeAlert situation. Although an alternative (and extremely dwarven) solution was just setting the surface on fire, with a few elven merchants included for good measure. Note to self: build future trade depots out of lignite or charcoal.
But this has begun leaving the realm of worldgen cookbookery. All the same, a few common desires for worldgens can be accomplished with various minor tweaks to the raws. The [RELIGION_SPHERE:DEATH] addition to dwarves in entity_standard.txt is probably my recommendation for least intrusive for the most benefit, followed by setting a MAXAGE for goblins in their creature file.
So back on topic, how do I add more rivers to my world with World Painter or PerfectWorld? I've got high rainfall over mountain tiles and their surrounding tiles, a high number of river starts, and low drainage in various directions around those mountains, but very few actual rivers and brooks.