In 100 years (consider the fact that larger nations will need to get their shit together, the space required to settle prior etc), San Marino can reach a quite decent population of 1,6 million, if maximal population growth is present (0,04). Plus, I don't know if their present land has enough carrying capacity for current population. I see them in the end controlling Marche and Romagna. Plus, they would establish contact with Switzerland quickly and I guess many Italians from Switzerland might want to live in an Italian state, especially if Switzerland centralizes and becomes German-dominated, which could easily occur during the initial chaos. If they don't, then I see San Marino joining Switzerland, but it's canton territory being greatly extended since who would be insane to make a completely new administrative apparatus out of scratch when they got a perfectly fine city already made