The current transitions between biomes is too drastic. in just a couple of metres, the terrain will visibly change from one biome to the next. It should definitely be more gradual, like over 50-100 metres.
Current rivers are just stupid. They don't flow, and make no sense. The rivers need to start in higher terrain, and flow down to the sea, or a lake. And 2 stream/rivers joining together should make a wider river.
The flowing river should then allow us to build water mills, in order to power basic machinery, such as a mill to grind wheat into flour, or to pump water through a pipe, to irrigate a crop, or transfer water to higher ground such as a pond or fountain.
It's not a limitation in coding. There are already mods that do each of these things, but they are separate and can't be used together.
The rivers/streams mod uses it's own worldtype, which means you won't get the other fancy terrain from another terrain mod. And it's unlikely they will be compatible with the best tree mod, Dynamic Trees. This is why such improvements must be added to vanilla; so they are compatible.