I can recommend IST for Sept (ck for season ending dates now). Amazing destination. I can assist w/hotel info.
Malta--very small, very historical, good food and easy to navigate by bus. Relatively cheap for digs--lots of variety for accoms. End Sept is good weather. We just did it. Easy to get from Paris via zeds. Plan to return this summer or fall.
Asia--HKG is excellent option but hotels can be $$. Ck for Sept holidays/festivals.
Thailand--always a fave for us but can be tricky w/out S2 but good to access w/zeds from SIN or HKG as needed.
Guess it all depends on how much time you have.
We've always wanted to go to Croatia so that's prob next year.