Something to explore.
Mainly redpen for early wireframes and prototyping.
A lot of Google Docs and Google Slides (not open source but I am fine with choosing the best tool not necessarily the most open).
There is of course this Discourse but it seems social platforms even those for GTD are moving more to a stream, but I think discourse->github could work.
The goal has to be to move people into github rather than assume they meet you there. Every team needs to think about how do we identify core contributors that might be ready to level up to github.
Github sucks plain and simple for non-devs. The langauge is unfamiliar: merge, commit, r, pull requests. Ohh nd you have to open terminal which makes it an immediate non-starter for most people.
What folks built with GitDone has potential, but be prepared for people like me, new to github, blogging through the issues, hijacking issues, and flooding it with +1 and emojis. You know the kind of stuff you see power github users complaining about all the time.
I also like the idea of people doing much of their thinking on their own blogs and syndicating back to the team through a planet or stream. We should be very explicit in baking story telling into participation software lab…
If the software lab is looking for an open source streaming software I have been loving Known.