+1, cool idea
Suggestion: Add an indicator for the first position in a game you haven't seen before in game Review

Sounds like a pretty interesting idea. So this idea would pop up every time you analyze a game? Or would it be a feature you would enable?

This kind of thing doesn't seem particularly hard given what "game review" already does. At the end of book moves it says "this is the last book move, it's QGD, and you've played it 32 times with a win/loss rate of xyz". So, clearly, the engine is already searching your personal game history.
So, it would seem to be easy to add this one, too -- a note that says "you've never reached this position before" or something like that. (Is that what you're asking?)
A sort of personal novelty popup in game review would be extremely helpful. I know that I would benefit greatly from it, as I struggle through memorizing theory, it'd be nice to see when I'm leaving my known lines. Could also make an interesting insights statistic (average number of moves until personally unknown position).