Another "limited opportunity" that I've seen river cruise itineraries based on is the Oberammergau Passion Play and it looks like that will be 2022 as well....(every 10years?).

Also would add - the Rhine & Danube are "easy" rivers for cruises and navigation. Anything in Italy will be limited time on the water (or rather, limited distances traveled?). Uniworld has some fascinating Venice itineraries but I don't really think of them as "river cruises". Similar with the Douro I believe. And as I alluded to, high water and low water cause navigation problems for river cruises, just another thing to consider when booking this type of vacation.