1. *Premix Routing* is a layer of "onion routing" at the beginning of a request, to protect against correlation attacks.
This will be difficult to implement, the best option is probably to divide the darknet into cells, groups of say 50 nodes within which the topology is published, and choose 2 random nodes from within the cell (with each node in the cell equally likely to be picked), to tunnel through.
Then your anonymity set is the cell.
We cannot just choose a node and then a node connected to it, because if the first node is evil, the second node may be fake.
Choosing two nodes at random from the cell prevents predecessor-probability attacks. To prevent fraction-of-a-splitfile attacks and node downtime attacks, we need the cellular structure.
Note that the above is for darknet. It's likely that we will need a completely different mechanism more like what I2P or Tor does for peer selection on opennet.
2. We cannot practically use onion routing on Darknet, but rendezvous tunnels may achieve something similar, by sending a bunch of (partially) random routed anchors out and having them meet up and use a shared secret scheme to establish a tunnel. Hence expertise with tunnels can help Freenet in the medium term. As it will slow things down we may only use it for those blocks which are predictable, or on those nodes with the strongest security requirements.