With a pod after 1900, the easiest seems to be Italian participation with the Central Powers or Axis followed by winning [especially the former], then have a separatist revolt that gives Sardinia independence with Italian acceptance. If temporary, the Italians taking over a temporarily independent and short lived Sardinia should do the trick. Or an extremely pyrrhic victory or stalemate in WW1 with a Central Power Italy losing Sardinia and gaining Corsica should be possible, but Sardinia might be in French hands to safeguard the Mediterranean and as a compensation. Otherwise, why won't the French and British take over both Corsica and Sardinia [is it impossible]?