I’m from the EU and my budget is about 3500 euro including flight(s).
Some countries that are on my mind: Colombia, Thailand, Indonesia, Ghana, South Korea, Japan.
Do you like rain and cold? Norway! Do you want more rain and it to be even colder? Northern Norway! We got it all, rock, grass, water. You name any of those three. We got 'em baby. Tall rock and deep water, we got it all.
I was thinking of a little bit more sun. I will visit Norway when I have won the lottery.
Vietnam in October is fairly nice in the north. Starting to cool down at night a bit. Spend a week to ten days in Vietnam going from north to south (Hanoi, Halong Bay (and possibly Ninh Binh), Danang/Hoi An, and Saigon) and then then 3 days in Siem Reap, Cambodia to visit the Ankor Temple complex and then a few days in Thailand (Bangkok and the nearby Ayuthaya temple areas and maybe fly down to Phuket or take a bus to Pattaya).