How are clients expected to know what parameters to use in the domain separator? This is easy when signing for a particular contract with a known domain separator, but there is no standard mechanism for use cases where it is not known beforehand.
For a concrete example, people have run into this problem in EIP-2612 (ERC20 permit). Some tokens include the version parameter (OpenZeppelin, USDC), others don’t (COMP), and those who do may use different values (OpenZeppelin: default "1", USDC: "2"). There is no way to query the contract if version is used and with what value. EIP-2612 includes a DOMAIN_SEPARATOR getter, but it would be wrong to use this value without verifying that it corresponds to the current chain id, for which all the parameters are necessary.
I imagine any other EIP that builds on EIP-712 signatures will run into the same issue. The only solution right now is to make it a part of those EIPs what the domain separator is supposed to look like, but I think it would be better to include a standard mechanism either in EIP-712 or in a complementary EIP.