Growth in battery capacity seems to have stalled but one thing keeps growing - the rate at which batteries charge. Qualcomm is rumored to introduce Quick Charge 4.0 soon which will push a whopping 28W of power to supported phones.
For comparison, current Quick Charge 3.0 tops out at 18W, Huawei’ Super Charge is faster at 22.5W, Oppo’s VOOC is at 20W. Google is reportedly considering pushing manufacturers to adopt the Power Delivery standard for phones that use USB Type-C - Google’s own Pixel phones support it, but they top out at 18W (technically, the standard can do up to 100W, but actually doing it on a phone will be tricky).
Quick Charge 4.0 is said to offer charging at either 5V (4.7A to 5.6A) or 9V (at 3A). There’s no mention of 12V or 20V options which may be a good thing - charging at a high voltage is the easy way to up the power, but down-converting that to a voltage a lithium battery can accept creates a lot of heat.
You can look at Huawei’s Super Charge and Oppo’s VOOC - they stick to a low voltage (5V) and use a high current (up to 5A). Super Charge can even drop below 5V to remove the need for voltage conversion and the heat that comes with it.
Quick Charge 4.0 is said to feature INOV (Intelligent Negotiation of Optimum Voltage), which should deliver the the best voltage for the battery. QC 3.0 can go between 3.6V to 20V in 0.2V steps, INOV will presumably improve on that.
a dual hot-swap battery will be better. people wont have the need to charge their batteries in a rush. leisurely they can plug-out the secondary battery and charge it whenever they want. safety is important than features...
Good news, welcome
It should be mentioned that Google does not want to support Qualcomm specific extensions to fast charging. http://arstechnica.com/gadgets/2016/11/google-threatens-qualcomm-quick-charge-with-android-incompatibility/ All hail the standards proces...