How would a P2P "Collective Transaction" be implemented (to increase privacy)
Dark Wallet is probably the closest to actively implementing something along these lines in a client.

See:

https://wiki.unsystem.net/index.php/DarkWallet/Alpha#Mixing


  When sending, the transaction is sent to an internal mixer, that
  starts announcing on the coinjoin channel, if another peer answers,
  both will negotiate a shared transaction that includes both peers
  inputs and outputs. If successful the final transaction will be sent
  into the network.
  
  You can see how a transaction from the mixer looks
  https://blockchain.info/tx/c38aac9910f327700e0f199972eed8ea7c6b1920e965f9cb48a92973e7325046


And also the multisig section that follows:


  Multisig funds are special bitcoin addresses that can be spent from
  only by a number of participants signing the transactions. This means
  accounts can be held among several people or devices, and several
  signatures (above some threshold) will be needed to spend.