Here is an idea to reduce the damage from competing blockchain-forks: each transaction is required to commit to a recent block:

Third bullet point on:

https://en.bitcoin.it/wiki/User:Gmaxwell/alt_ideas

• Transaction block commitments. Each transaction (or signature?) should contain a block index and 32 (?) least significant bits of the block hash. The transaction's fees are only valid (or only their full value?) if they are mined in a chain they agree with. This would let people making bitcoin transactions 'vote with their wallets' on the identity of the chain they consider important. This isn't a viable POW replacement, but would greatly reduce the economic benefit of mining moderate depth forks, esp as fees begin to dominate the block reward. "You don't get (all) of my fee payment if you are mining a chain I don't like!"
  • Nodes would typical checkpoint a few blocks in the past from their current height to avoid overstating their opinion unnecessarily.
  • Deep checkpoints could be automatically triggered by observing a critical mass of coins-day-destroyed confirming them— creating a PoS-ish system, though this is subject to the 'nothing at stake' problem of PoS, and is probably very dangerous. (e.g. isolation risk for newly bootsrapping nodes)