As a friendly reminder, SRV records exist and are great at fixing that magic port syndrome (unless you were hinting at the infinite corporate firewall appliances, for which I have no magic fix)
Right. Egress on anything other than tcp/443 is probably a non-starter for any new protocol.
The question I was alluding to is: if it's HTTP-ish over tcp/443, wouldn't it still be the web anyway?
But thinking about it more, the server could easily select a protocol based on the first chunk of the client request. And the example of RTP suggests that maybe even TCP would be optional.
But it seems apparent to me that it will have to work over HTTP/QUIC, and TCP port 443.
Which prompts the obvious question ...