Comments - Connecting
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I think that an extension like you describe might make sense, but there needs to be a consensus, and server should signal the client that it understands whatever client sends, once it understands it. Usually, capability bits are used for this kind of signal, but MySQL is running out of these bits, and needs some extension.
Otherwise, I believe any form of utf8mb4 is good enough for majority of clients and their use cases, e.g utf8mb4_bin. The connection's collation only affects comparison of literal strings, which is not what everyone would need from database, and for rare cases, SET NAMES could be appropriate.