Often, there is a country I want to visit by bicycle. When I don't have one of those, I look for Unesco World Heritage Sites that offer stunning scenery/experience and plan a tour around it/them.
I have been learning Italian and now speak it reasonably well. So, should international touring again become possible, I look for tours that spend time in Italy. Given it is pretty much in the middle of Europe, it isn't hard to find a place to go that includes some part of Italy.
Once I have a destination, I begin researching the various routes people have ridden over the years in that area. I read journals, check out routing sites, write to bike clubs in the area, look for websites. Then, I start mapping my route, looking for places to stay each night as I don't camp. My limit for now is 50 miles and 4000 feet of climbing. But, I'd prefer to stay below both of these and still cover a fair amount of ground.
My last planned, but cancelled, ride started in Puglia (southeast Italy) then a ferry to Dubrovnik and riding up to the Durmitor National Park in Montenegro and then returning back via Bosnia. I still hope to ride this route some day.