Would be neat to see a mesh-network of these running on old phones of which most people have 1 or 2 of at their house with a REALLY simple installer.
A few of these sprout up organically in locales and it would be like the BBS days of yore - ultimately it could lead to a larger geographical footprint of a "side-net".
mesh is in preparation.
There is already a mesh implementation done for the sister-project "LibraryBox. The code is contributed back to PirateBox. It still needs some tine addons to work nicely.
Bridging could be done with some configuration, I think. IMHO the 2nd device don't need to be a PirateBox.
Check out "The Serval Project" - it checks a few of your boxes, although it focuses more on securing and expand 1-to-1 communication than the community/BBS aspect.
A few of these sprout up organically in locales and it would be like the BBS days of yore - ultimately it could lead to a larger geographical footprint of a "side-net".