Red hints annoy me honestly, when I meet these in real editors. I was thinking of something like grammar walker that completes input to shortest finished constructs. While not semantically correct, but syntactically like yes.

  pr
  pr;

  printf(
  printf(<?expr?>);

Or to not edit code as text at all, but as a tree. Like apple plist editor or windows registry editor, but with convenient shortcuts to add constructs. Xcode can sort of simulate this with templates and placeholders:

  if␣
  if (<?condition?>) {
      <?body?>
  }