I’m very sorry if this has been discussed before: From a conceptual standpoint, I would prefer if this could be a precompiled contract, since it is external to actual computations in the EVM. Until we reduce the gas costs for calls to precompiled contracts (I would very much like to include this in the next hard fork, if possible), using an opcode is probably the only practical possibility.
If we “waste” a full opcode for this, the let’s at least make it extensible in some way: Make it take a parameter and return the Chain ID if the parameter is zero, the Hardfork number if it is one, and so on and also define reserved and non-reserved parameters.
Would that be a good suggestion?