Not in my wildest dreams could I have imagined that finding a diary would have such fortunate consequences.

not/never in one's wildest dreams 是固定短语，意思是"做梦也想不到""完全出乎意料"，用于强调某事远远超出预期。其中 one's 是物主代词，可根据主语替换为 my、your、his、her、their、our 等。