In OTL, after Italy signed the Armistice of Cassible with the Allies, Germany immediately invaded, taking over about 3/4 of the Italian peninsula within two weeks. It took almost two years of hard fighting to drive the Germans out.
My understanding is that some of this was due to misunderstandings and diplomatic issues between Italy and the allies. Is it plausible to have an Italy that successfully resists the Germans long enough for US and UK troops to reach the north of the country, or else to have the Allies land in the north directly? If so, how does this affect the rest of the war. The Alps are pretty tough to cross, but could you have, e.g., the Allies forgoing Overlord and attacking France by land instead?