The immediate bytes decoding with the [0x5b–0x7f] forbidden range (current spec) can by significantly improved by using different normalization mapping: consider the byte to be a signed 8-bit integer.
Proof of Concept: Compiler Explorer
I can send the spec update. However, it is unclear to me what is the status of Update EIP-8024: Switch ecoding to push-postfix by frangio · Pull Request #11094 · ethereum/EIPs · GitHub.