I just found that the Emperor Friedrich II stood in the way of an attempted marriage between St. Agnes of Bohemia (b.1211) and Henry III. It was a complicated saga in that Agnes was intended for Friedrich's son, Heinrich (VII), and was sent to the court of the Babenberg duke to be raised. The Babenberg duke pushed the agenda of his own daughter, Margarethe, to marry Heinrich (which later happened); since I imagine he wanted Agnes to marry one of his own sons (Leopold (b.1207); Heinrich (b.1208) or Friedrich (b.1211)). And the imperial betrothal was finally cancelled in 1226 (same year as Henry III's Breton engagement). Agnes' dad went to war against the Babenbergs for this, and attempted to betrothe Agnes to Henry III of England, but the emperor blocked this, since he [Friedrich] wanted to marry Agnes himself. Agnes told them to shove it and she removed herself from the equation.
So, should Friedrich's second wife still be alive and kicking (I have it that the Queen of Jerusalem only died in 1228, so I'm not sure why Friedrich would want to marry Agnes in 1226 - unless he wanted to marry Agnes instead of his second wife?), would the emperor block said betrothal? And what might the difference of said marriage be? Where would Eleanor of Provence marry instead? To Richard of Cornwall (instead of her younger sister Sancha?), or, the Count of Toulouse (who asked Blanca of Castile, Regent of France to help him win one of the Provencal princesses' hands, but she cock-blocked him and married them to her sons instead)?