The main question is which P2P overlay network to use, if any: the prior incarnation used Pinecone (a variant of Yggdrasil), whereas this time we're pondering keeping it simpler and more scalable and using Matrix itself as the backbone to connect together smallish local P2P meshes - so by default you try to route via Matrix, but failing that you look on your LAN or BLE to see if you can talk directly to whoever you're addressing. Time will tell if this works :)