What if Matilda of Canossa marries into the Welf/D'Este family what would be the result of such marriage?
She did: in 1089 she married Welf II the Fat, oldest son of the duke of Bavaria. It was an obvious political marriage (Matilda was 26 years older than Welf) and it is doubtful the marriage was ever consumed. Since the Great Countess had already secretly deeded all of her properties to the church, the marriage had little or no future, and ended in 1095 when the house of Bavaria mended up the fences with the emperor.
It would have been more interesting if Matilda had married either the father of Welf the Fat (another Welf, unsurprisingly) or his younger brother Fulc (both were sons of Azzo d'Este) but it would have meant a complete breach between the house of Este/Welf and the emperor.