This page is to record changes to Bitcoin that might be desirable, but that will require a "hard" block-chain split (everybody must upgrade, old software will not accept blocks/transactions created with the new rules).
This page is *not* for changes that can be accomplished in way that is compatible with old software.
Changes to hard-coded limits
- Replace hard-coded maximum block size (1,000,000 bytes) and maximum number of signature operations per block (20,000)
Major structural changes
- "Flip the chain", instead of committing to new transactions, commit to the summaries of open transactions:  
- Increased efficiency for merged mining: restructure the primary header to make the bitcoin specific data non-mandatory. (e.g. the block chain specific stuff would go into second header connected by a header tree), making the primary headers pure timestamps.
Transaction behavior changes
- Improved signature types to allow for partial malleability of outputs. (e.g. make it easier to add a fee onto someone elses transaction, or to take fees from a transaction without outputs set aside for that putpose)
- Pervasive ECC public key-recovery to reduce transaction sizes (can be done partially without breaking compatibility completely)
- Support for a post-quantum signature scheme. Lamport signatures have nice intuitive security properties, but it and all other similar schemes have extreme space requirements that would require structural changes to the blockchain to accommodate. Additional signature types could be kludged into the existing system with script extensions but would be better supported natively.
Please don't list anything here which would significantly change the committed overall economics of the system, it's safe to assume anything with significant economic impact will _never_ be changed in Bitcoin, because such changes would undermine the trust people have in the system, though they may form the basis of an interesting alternative chain.
- Increase currency granularity: three more decimal places would still fit all Bitcoin in 2^62 (e.g. you could sum any two signed bitcoin values without overflow in 64 bits) and would allow for pico-btc.
- Byte order consistency
- Eliminate redundancies in the variable length integer encodings
- Avoiding hashes covering malleable fields
- CHECKMULTISIG popping one-too-many items off the stack
- difficulty adjustment periods should overlap (prevent potential 'timejacking')