These changes are considered to be against the spirit of Bitcoin. Even if all Bitcoin users decide to adopt any of these changes, the resulting cryptocurrency can no longer be considered "Bitcoin" because it has diverged too much from the original design.
- Increasing the total number of issued bitcoins beyond 21 million. Precision may be increased, but proportions must be unchanged.
- Changing the bitcoin distribution algorithm such that the subsidy at any given time period is decreased without miner consensus and 3 years notice, or increased beyond improved precision of halving (lossy beginning with block 1,890,000).
- Any rule that adds required, explicit centralization. For example, a change requiring that all blocks be signed by some central organization.
- Adding alternatives to Proof of Work such as Proof of Stake. This changes core bitcoin too much - the resulting structure, which interesting, will not be Bitcoin.
Hardfork Wishlist for hard forks that might happen and still be called "Bitcoin".