Formal:
* The future of the company is an imponderable at this point, with so many factors in play.
* The impact of climate change on global agriculture is an imponderable that must be addressed with urgent action.
Informal:
* Trying to predict the stock market is an imponderable – even the experts get it wrong sometimes.
* "Should I take this job offer?" she pondered, realizing the answer was an imponderable that only time could reveal.
Figurative:
* The sheer immensity of the universe makes the question of our place in it an imponderable that leaves many speechless.
Let me know if you'd like more examples or have any other questions!