The Omnicharge Omni 20 is a 20,400mAh portable battery capable of delivering up to 100W via the AC outlet. This is the only external battery that can not only power the 87W MBPs (15-inch) but also charge them during use (an important factory when shooting on location). The only downside here is that you would have to carry the full AC charging unit along with this Omnicharge external battery.
This leaves plenty of room in the market for an external battery that could make use of the high-powered USB-C ports for charging without needing to lug around the Apple AC adapter for the computer. If somebody would put this type of battery out with a high enough capacity, I guarantee you at least a good number of sales in the photo industry alone.