Hi! There are quite a few tools out there that can help you with your predicament, let me walk you through a few I know of:
- Trello: Its great to create a roadmap for the project in which you can outline stories or epics and what each one of those will contain.
- Miro: As GoingAFK mentioned, I also used miro a lot to create flowcharts that better explain my game design on a high level and makes it that much easier to understand and digest.
- Machinations: a bit more complex, this tool allows you to distill entire systems and actually see them working, really cool for things like loot systems or level up systems.
- Canvas: its an alternative to miro, works in a very similar fashion.
- Jira/Clickup: Great for tickets and coordination if you are working with a team.
- Diagrams.net: is another free tool for diagram creations.
- Google Docs/Slides: don’t underestimate a good design document created and shared through docs and slides, it could save you a lot of headaches early on to create a comprehensive design document/bible for your game.
I hope these helps!