The Great Countess (Magna Comitissa) is possibly my favourite historical figure: she signed an age, the age of the fight for the investitures and of the birth of the free town, the comuni: last daughter of a dinasty who claimed descendance from the Langobard dukes of Tuscia and the gratest Frank nobles, niece and cousin of the German emperors, strongest bulwark of the papacy in its fight against the empire. She was as successful in her role of feudal lady and patroness of the arts as she was unlucky in her family life (her father killed when she was 10, her only known daughter died just after birth, a very unhappy marriage with Godfrey the Hunchback of Lorraine, a worse one and never consummated with Welf the Fat of Bavaria. Recent biographers ascribe her unfortunate lovelife to a lifelong relation with Ildebrand of Soana, who became Gregory VII and whose sword and shield the great countess was always, against the empire and against the great Roman nobles, but in the end the legacy of Matilde did not survive her death).
I contributed to a Mathilde TL a few years ago (look up Tuscan Sons, by ShadowKnight): the POD was that the marriage between Mathilda and Godfrey was fruitful, two sons and two daughters, and there was no phisycal relation with Ildebrand (Matilde found love in the arms of Roger Curthose -who met her IOTL and actually proposed to her - had a daughter from him who later became duchess of Normandy and ultimately went to the 1st Crusade with Roger and died at the taking of Jerusalem). Politically she was even more successful: she befriended the cities against the feudal lords and the support of the cities allowed her to expand her domains even more. Her firstborn, Bonifacio (named after her father) became king of Italy all but in name. her second son was duke of Spoleto and carved a state in Outremer.
It was quite a good TL, if I say so, and I suggest you to have a look.
Today I'd do it possibly in a slightly different way: maybe the POD would be her father not being killed in 1052 during a hunt near Mantua: this pod would most likely butterfly away the early death of her brothers and would result in a strong feudal state in Italy (maybe too strong: even if it is not proven, it is most likely the Bonifacio of Canossa - nicknamed the Black or the Tyrant - was killed on orders of Henry III who was suspicious of this too rich and powerful noble who had married his sister Beatrice) and quite possibly into an early end for the HRE influence in Italy. A Bonifacio's POD would be simpler to manage (the weakness of Matilda was that the lands the Canossas had in fief from the empire could not be inherited by a woman) but would butterfly away a great historical figure.