1. Is it possible to re-use part of diagram, like require statement in PlantUML?

2. Any plans for native C4 model support, or examples how to emulate it?

3. What about layout control? By default D2 generate way nicer layout, but it is synthetic. How I can tune it, give some hints, etc?

2. with any integration, if there's enough demand, we'll do it. Once we add interactivity to diagrams, it should be trivial to emulate. However I should mention that Simon Brown (the creator of C4) has his own DSL for C4 https://github.com/structurizr/dsl)! We have no intention of competing with it.

3. Currently the options to tune are limited. The Terrastruct app has granular layout control via drag-and-drop, and we're working to bring some of that to native/open-source.