Proposing an alternate of guard named guards that implicitly returns out of the function and would supply nil if an optional return was declared. This would be purely for the sake of brevity yet still allow for the more explicit guard version to be used.
Changes to guard (or variations on it) were once a frequently mentioned topic. They've not been successful, and with the very high bar now for additions to syntax, it is unlikely to meet with success in the future.
If this family of ideas has not already been added to the list of "frequently rejected changes," then it should be for future reference.