The Mi Max 2, which is likely to be announced tomorrow, will have a plus-sized battery and reverse-charging OTG capabilities.
https://www.notebookcheck.net/Xiaomi-Mi-Max-2-to-come-with-massive-battery-and-OTG-support.223546.0.html
It's hard to find anything better than this in this price range, other than the upcoming Lenovo P3. Really hope they've upped the build quality and fixed the OS bugs from the original.