Apple has already announced that iPhone fall event will be held on 9 September, at the Bill Graham Civic Auditorium in San Francisco. You can also ask Siri to give you a hint, and Siri will tell you about this fall even event. Apple will introduce iPhone 6s and 6s plus in this event. There is also rumor about iPhone 6C, but Apple may not announce 6C in this event if they discontinue iPhone 5C.
iPhone 6S and 6S Plus would feature force touch, and the leak suggests that this feature would make the device thicker and will reduce the Battery mAh.
According to latest leak, iPhone 6s would feature 1715 mAh battery compared to 1810 mAh found in iPhone 6. iPhone 6s Plus would have 2750 mAh battery compared to 2915 mAh in existing model.
Decrease battery mAh may or may not have less battery life. Optimized and imrpoved pefrormance may help the device to run longer even with less mAh battery. You can take the example of Galaxy Note 4 and Note 5. Galaxy Note 5 runs around 30 mins longer compared to Note 4 even with 220 mAh less power. However, iPhone 6 already has smaller battery, and Apple shouldn’t have done this (if leak is true).
Apart from this, iPhone 6s and 6s plus has been rumored to have 12-megapixel camera that would shoot 4K video, 14nm chip technology, A9 chip with improved performance and more GHz, force touch, more color options etc..