Can you help please?
Which is better?
"We cannot foresee which / what could be the effects of such modifications."
I think both are correct but when you use "Which" you are emphasizing the kind of effects the modifications are going to produce and using "what" you are referring more to the consequences of such modiffications.
I'm not sure if my explanation is very understandable
It's similar to the collocation "I wish I may". 'may' is too high on a probability/certainty scale to be generally used; it doesn't easily collocate with 'wish'.