Hmmmm....
Sceletium tortuosum aka Kanna is fairly heat tolerate and has some small but very pretty flowers which might make a nice accent to the more common garden flowers.
Salvia spendens may also work for your larger baskets.
A basket of Morning Glories in Heavenly Blue, Flying Saucers and Pearly Gates would be a visual feast.
Passion Flower is a woody vine so may not take well to baskets, but if you can get it grow and flower is very striking.
A plant I've rarely seen cultivated for its flowers is cat-mint. The one person I know who had them here in Colorado had them do great, even in brutal summers.