The Caves & Cliffs update removed the previously common aquifers, this greatly reduces the number of waterfalls and creeks Water Erosion can create. Add more water in the hills and mountains! Maybe use the ore generating system? https://fabricmc.net/wiki/tutorial:ores Or do it entirely custom.

I can't remember what the aquifers were like in the old worldgen, but you could just add some of the available features, like "lake" or "spring," and just set them to generate above a certain Y level.

I am not a developer, but I have used these generators to make my own features (for example burnt trees,) and then used a Forge biome modifier json to add them to existing biomes. Relevant Fabric page.

Better Default Biomes seems to have some examples in the source, like adding palm trees and stuff.