I think that the solution is clear, make a language server for the command line. Each and every single command line interface is its own language so they all need their own language server that can provide meaningful completions.
Then bash and other shells can add support for language servers and we have the problem solved.
I imagine this to be a little bit over the top in most cases. A standard flag like --complete-rahyeiB1 (with a sufficiently long random string to avoid collisions with existing flags) which takes the already written argument list as an argument and prints possible completions would be nice though.
