It's not clear that one can evaluate the probability of an unknown problem, and all these certification processes work by evaluating probabilities.
The 737 design actually has no
unknown problems, not any variant from the original to the NG, they have more than half a century of active operation - but the MAX does have problems, known and unknown, and these problems all
stem from engine size and placement.
It'd be nice to have plans A-Z for every contingency, but that's not how things work in aviation.
That's right, and because of that in civil
aviation people do their darndest to design crafts to have as few contingencies as humanly possible.
It is a very conservative business, for that very reason. It's impossible to have plans for all contingencies, thus creating or engineering situations which present new contingencies is simply not encouraged. It's bad practice.
No matter how you look at it, the engine size and placement on the MAX is what created new contingencies, which simply did not exist before on the 737.
These were known
contingencies, obviously, since steps were made to remedy them. These remedies were quite inadequate, which is why it's grounded worldwide.
Not because of "every contingency" wasn't accounted for, but as a result of a known contingency being addressed in an inadequate way. And addressing issues in an inadequate way is not how things work in aviation.