They could use one of the steeper ascents of Etna. Like the one to Rifugio Sapienza from Zafferana.With Etna and Gran Sasso d'Italia the outcome is quite clear all the time.
There aren't many tougher climbs than that in the Apennines. Maybe only San Pellegrino in Alpe, Blockhaus and Nerone. But Giro should really have a climb with gradients of at least 7-8 % over 10+ kms before the first restday. And a big(ish) medium mountain stage. Especially when the don't reach the Alps or Dolomites until stage 13 like this year.