Queen Berengaria was the eldest daughter of Alfonso VIII of Castile and Eleanor of England. Originally she was married to Konrad II, Duke of Swabia, but the marriage was later broken (according to wiki, Berengaria was influenced by third parties - like Eleanor of Aquitaine) and Berengaria married Alfonso IX of Léon instead, while Konrad was later murdered in Germany.
So, what if Berengaria was to marry Konrad? Hohenstaufen Castile perhaps? Or would a change in 1191 mean that her younger brothers' can avoid their fatal fever (and perhaps get a Danish bride (although while they speak of a betrothal between Prince Fernando and a Danish princess, I can't find any without a major age difference)) or a falling roof tile (Enrique I). And would/could it remove Konrad's murder (after reading his wiki, I wonder how happy the marriage would've been)? Would Konrad settle down after marrying? Could we see an earlier king of Castile claim the imperial purple?