I describe this as Fish-like as it is referenced as such by zsh-autocomplete, and I would define it as follows:
The appearance of a (temporary) automatically inserted completion match, that is highlighted in greyscale. Only when accepting the match the cursor position would be affected and normal highlight groups would be applied.
I think this concept of "phantom text" would be a nice extension to the naitive completion options. It would be a means for (future multi-line) completions to be inserted when selected from the menu, without moving the cursor or changing the view state of the window, depending on other completion options. I think this is doable with the asynchronicity these days.
Do you know if there have been efforts made to do this? Do you this is useful?