If I recall correctly Vitalik decided to get rid of crypto-opcodes (like SHA256). The point was to make Ethereum language more universal. R u going to do the next step and get rid of 256-bit numbers? Their usage don't make much sense coz 512-bit cryptoalgos will require to use different tricks to work with 512-bit numbers. Could we have only 64-bit numbers, this would help a lot to implement Ethereum interpretator in Java (which doesn't have native support of 256-bit math).
Edit: Maybe 32-bit is the best choice, JavaScript guys would be happy too.
The hardest ECC scheme (publicly) broken to date had a 112-bit key for the prime field case and a 109-bit key for the binary field case. So 64-bit obviously won't do and at the same time it's a long way until 512-bit keys will be needed, if ever.