Given the state of italy at the time post WW1, i think one extreme would have eventually taken over, I'm sure if Fascism never takes hold, their would be funding for a communist revolution provided by the Soviets (especially if Trotsky had anything to do with it) or even without it, i'm sure someone else would have. Needless to say, Italy allies with the Soviets in WW2 assuming the enemy is still Nazi Germany.
If its just not Mussolini, i'd imagine someone would have taken up the fascist cause. Depending on the type of Fascism, they could ally themselves with Hitler or just stay out of the war completely.
I'd say another option is a Nationalist Italy, one that isn't necessarily full blown fascist perse. This country may even end up teaming with the allies in truth.
Italy wasn't in a good state, so i'd imagine some kind of extreme view would have prevailed just due to the people probably being desperate and wanting something to drastically improve their standard of living.