I doubt Caroline will let them marry outside Monaco
But if they will, they could choose among lots of properties owned by the Borromeo family throughout the Italian North, especially in Lombardy where they settled after leaving Tuscany where they had originated from. Maybe these could be wonderful locations for a princely wedding. Not in the summer, though: lots of humidity and mosquitos!
homehtml (scroll down and click on Foto Tour, then click on each little hand appearing on your screen to visit the Villa)
Castello Borromeo di Peschiera - Visite
Or here, in the villa Trissino owned by the brother of Bea's grandfather in the Veneto Region (Italy's North/East). Bea's mother Paola is one of the daughters of Count Pietro Marzotto.
Villa Trissino Marzotto a Trissino « Mimma Pallavicini’s Weblog
(click on the pics to enlarge them)