What unthinkable idea do you think is worth considering?
Reference: https://www.youtube.com/watch?v=Z9i_16eKvdc&list=PLUX-sja_eqhTwFEdyu9bBTeWgo3FTxFDz&index=1
Keep in mind that the existence of extraterrestrial life, and by implication some amount of ET intelligence, is an explicit scientific prediction. This prediction is even more solid now that we've identified hundreds of extrasolar planets in habitable zones, many of which orbit stars older than ours. We have no good explanation for why we don't see aliens already even though we probably should, an observation known as the Fermi paradox.
It's worth thinking about what effect this might have on culture, philosophy, religion, economy, etc. To give just one example of a huge implication: all of biological and human history would have to be re-evaluated from the perspective that the Earth is not a closed system.
Contact by radio should also be considered and is probably more likely, but physical contact either today or in the past would have wider consequences.
https://nickbostrom.com/existential/risks.html
As a side note, and from someone with negative karma, you may wish to peruse my recent comments about Covid, some seem to think they are unthinkable ideas while I persist in saying they are worth considering, although it is about an ongoing black swan and therefore probably not within the scope of your survey, they fit your description in that way.