Charles of Anjou had been King of Sicily for about 2 years when he defeated an invasion by the legitimate King of Sicily, Conrad II (aka Conradin), who was also Duke of Swabia and King of Jerusalem through his grandmother. IOTL Conradin was defeated, captured and executed.
What if Charles had lost and Conrad had won? Calabria, Apulia and most of Sicily has risen up in support of him.
Presumably Conrad becomes King of Sicily and the Pope hates him, Conrad then has to find someone to Rule Sicily assuming he wants to be King of the Germans, perhaps Enzo can be freed from Bolonga for the Job, Frederick of Badan, probably gets to advance his claim to the Duchy of Austria, Although Ottokar of Bohemia may have his own thoughts about that.
We find out if Conrad can hold the duchy of Swabia together. and if Conrad can stop the male llne of the Hohenstaufen becoming extinct. Maybe the Hohenstaufen get to renew their attempt to unite Italy
The Byzantine Emperor doesn't have to spend a fortune fending off Charles d'Anjou.
Peter of Aragon doesn't get to take Sicily which creates problems for the future Spanish takeover of Southern Italy, but on the positive side it would be unlikely that there would be an Argonese crusade. it becomes less likely that the House of Capetian-Anjou, gets to produce kings of Hungary Albana and Poland, including Poland's and perhaps Europe's only female king.