Of course you could always do better, but the specific feature mentioned in the parent comment (completing flags for "tar" instead of checking the man page) has been around for ages.

Regarding contextual analysis, the fundamental problem is that command execution relies on a list of strings (argv) with no types or meaning. Each command is responsible for parsing its own arguments, either manually or using a shared library (e.g. getopt). This provides loose coupling and flexibility, but makes introspection really difficult, and features like completions become an afterthought reimplemented in every shell either by special cases for each command or hacks like parsing help messages.

fish shell does exactly what you say, out of the box. c/p below:

  ~/.l/share: tar --<tab>                                                               
  --absolute-paths               (Don't strip leading /)
  --after-date                   (Only store newer files)
  --append                       (Append files to archive)
  --atime-preserve               (Keep access time)
  …and 58 more rows

As others have mentioned, zsh can be tuned to perform similarly. fish is nice in that if you don't have explicit completions for options set up it actually parses man pages.