If someone asks me "what's the least impressive thing you think AI won't be able to do by 20XX", I give answers like "make lots of original interesting math concepts". (See https://www.lesswrong.com/posts/sTDfraZab47KiRMmT/views-on-when-agi-comes-and-on-strategy-to-reduce#comments) People sometimes say "well that's a pretty impressive thing, you're talking about the height of intellectual labor".
A main reason I give examples like this is that math is an area where it's feasible for there to be a legible absence of legibilization. (By legible, I mean interpersonal explicitness https://www.lesswrong.com/posts/KuKaQEu7JjBNzcoj5/explicitness .) Mathematicians are supposed to make interesting novel definitions that legibilize inexplicit ideas / ways of thinking. If they don't, you can tell that their publications are not so interesting. It is legible that they failed to legibilize.
In fact I suspect there will be many much "easier" feats that AI won't be able to do for a while (some decades). Easier, in the sense that many more humans are able to do those feats. Much harder, in the sense that it requires creativity, and therefore requires having the algorithm-pieces for creativity. That's easy for humans because it's our birthright, but "hard" for AI because it doesn't have that yet. Lots of little novel "knacks", of the sort people can pick up; surprising connections or analogies; solving lots of little problems, or coming up with a way of seeing some sort of situation in a way that makes it make sense.
But knacks, insights, predictions, inventions--these are not categories. I'm not saying "AI can't make predictions" or "AI can't learn knacks", because that would be nonsensical, because those aren't categories. I'm saying that some things humans do, require broad-spectrum creativity; but it's hard to describe those things as a task, and so I don't give it as an answer to the question about what AI can't do. (If a task is stereotyped, not that hard, has clear feedback, and has lots of demonstrations--all things correlated with legibility--then probably AI will be able to do it soon!)
I agree that it's hard to operationalize; that's part of what my OP was saying. And then I think it's relatively easier to operationalize in mathematics, where it is in large part explicitly about creativity in my sense (but maybe not especially much in your sense). So that's where I'm getting my prediction; if you don't see what I mean by creativity, or wouldn't make the same prediction, then fair enough, we'll have to agree to disagree.