I know that the common structure is "If it had happened," but the thing is that some of the grammar books say that "If it had happened, we would have thought about it," is as correct as "If it would happen, one would have thought about it."
I don't agree. Which grammar book says that?
Conditional is not just restricted to the "have."
It's not, there are certain rules of how to express a condition...
Conditional 3:
I would if I could. (No "had") --> If I could cook, I would have done it.
Modal verbs (could in this case) can be used in conditionals.
If I would win the lottery, I would have told you. This is not OK, will/would is usually not used in a conditional clause, it's possible when you want to express willingness.
If I had won .... :up:
Not to mention the case of "wish."
Wishes in the present are usually expressed with the Past tense and wishes in the past with the Past Perfect.