Sequence diagrams are a secret weapon for designing any complex system. The y direction (relative time) actually means something, unlike most abstract diagrams. The x direction (actors) highlights the discrete set of participants. The arrows actually mean something - bugs and features and new problems all exist at interactions between actors and be systematically decomposed into work items, just find the arrows.

I know there are more formal tools (The P language comes to mind) but it's hard to beat the simplicity of markdown that gets rendered directly in github's UI.