Pruning design ideas
In 2016 the pruning concept has been designed to do one thing and one thing only. It is about removing historic blocks from a node based on a maximum disk-usage set in the configuration.
Effect of this is that this node can no longer serve any history because serving history is set as a boolean on or off using a service bit.
Random pruning mode
There is a large gap between the two current modes of everything (currently 75GB) and only what we need (2GB or so).
This mode would have two areas, it would keep a days worth of blocks to make sure that any reorgs etc would not cause a re-download, but it would have additionally have an area that can be used to store historical data to be shared on the network. Maybe 20 or 50GB.
One main feature of Bitcoin is that we have massive replication. Each node currently holds all the same data that every other node holds. But this doesn't have to be the case with pruned nodes. A node itself has no need for historic data at all.
The suggestion is that a node stores a random set of blocks. Dropping random blocks as the node runs out of disk-space. Additionally, we would introduce a new way to download blocks from other nodes which allows the node to say it doesn't actually have the block requested.
The effect of this setup is that many different nodes together end up having the total amount of blocks, even though each node only has a fraction of the total amount.
There have been talks before about changing the transaction format itself and specifically the way that signatures are stored. A signature is only needed at the time that a transaction is validated.
Network nodes that have been turned of for a longer time can choose to only download the blocks-headers and transactions so they can fully check them and update any wallets. This means that they can skip downloading of the transaction-signatures.
Any node that chooses not to check signatures on transactions while still checking the block for proof of work and general correctness would depend on the amount of proof-of-work that has gone into creating the whole chain to be safe in the assumption that many others have checked those transactions for validity. Naturally, any transactions in blocks less than, say, a week old still should be checked. This is purely about historic transactions.