My preference would be a way to include/exclude countries (especially if it can also account for border crossing times in some manner), but then past that group by high/low temperatures for that part of the year. It's one of those things that affects prep and packing a lot that most travel services don't give much insight into.
Thought about that. Seasonality and availability. You mentioned seasonality already. Availability is different. E.g. famous cherry blossom in Japan happens in Mar-Apr. That's is future plans. Thanks for pointing it!
