Ohhhhh where to start?
I'd love to go back to Greece (I just got back lol), and next time see more. I'd like to go on a shoulder season (May or September) for the heat but to avoid the tourists and crazy drivers, and rent a car and go up the coast and maybe back through the mountains (though those roads are fairly dangerous so I might want to bus it) then ditch the car in Athens and take ferries to some Islands, including Crete and Mycenae.
I'd love to travel through Italy. I haven't been, and I want to go everywhere. I love cities so Rome, Florence, Milan and Bologna are must-sees, but I'd also love to travel through Tuscany and along the Amalfi coast.
Spain/Portugal
The UK...rent a car, go through the countryside.
Road trip through the French countryside.
Stockholm
Japan
Australia
Brazil
Argentina
.....
That's all I've really thought of for now.