The RFC https://tools.ietf.org/html/rfc4106#page-5 states that, under AES-GCM IPsec, each packet is expanded by 8 bytes for the IV and up to 16 bytes for the ICV. Why? Since we already have a Sequence Number (4 or, in the case of ESN, 8 bytes), can't we just use that as an implicit IV? This saves 8 bytes per packet (which is sometimes negligible, but sometimes not), and doesn't seem to harm security at all - either way, we need to ensure that sequence numbers are unique. If anything, it's easier to ensure that one field (ESN) is unique, more than two?
If I am correct, and you can just the ESN has the implicit IV, is doing so compliant with the standards?
Note: I recognize this assumes only one encryptor per SA. For cases of multiple encryptors using the same key, each encryptor would need to prepend their unique id to it.