I'm actually surprised there isn't a thread like this, but I could just be looking in the wrong place.
Numerous engagements between important persons flew thick and fast in the past. Some were true, others merely rumor - and then you're left wondering 'where the Hell did they ever think that up' 0 but either way, some of them could've led to some pretty interesting changes to history as we know it.
So, please take part in the great what-if game of royal marriages:
I'll list some of mine:
Albert I of Belgium to GD Helena Vladimirovna of Russia
Oscar II of Sweden to Mary of Cambridge
Nikolai II of Russia to Hélène d'Orléans
Albert Victor, Duke of Clarence to Hélène d'Orléans
Max of Baden to GD Helena Vladimirovna of Russia
Duke Siegfried in Bavaria to Archduchess Maria Annunciata of Austria
Mary Tudor to Charles V, Holy Roman Emperor
Mary I, to Charles V, Holy Roman Emperor
Mary I to François I/François, Dauphin of France
Elizabeth I to Felipe II of Spain/Frederik II of Denmark/Erik XIV of Sweden/Henri III of France/François, duc d'Alençon/D. Carlos/Emanuele Filiberto of Savoy
Louis XIII of France to Nicole/Claude de Lorraine
Isabella Clara Eugenia of Spain to HRE Rudolf II
Catalina Miguela of Spain to Duke Téodosio of Bragança
What're your interesting failed/broken OTL engagements