I'm concerned as to how are we going to reach consensus if new opcodes are to be included on the EVM or if we would like to update the cost of some opcodes. The gas price has been chosen arbitrarily at v1. Is there any approach to build on-chain a consensus process for gas cost?
Ethereum Improvement Proposals (EIPs) would be where that gets discussed. Anyone can submit an EIP idea as an issue, or as a pull request if you already have it in a document following the EIP template.
There is no immediate need to have those discussions on-chain; blockchains reach consensus on the state of their chain, not the opinions of the people using said chain. Ethereum could of course be used for that, I'm just pointing out the common mistake of mixing up consensus between people and consensus on the chain's state.