When Leica release a new M a couple of years later they release the P version. In general its the same camera with stronger rear glass screen, no red dot and occasionally a bigger buffer.
Back when the original M8; the first digital M was released they released the M8.2 to address its short comings. Since the M9 the P has not needed to address any short comings just bring a few enhancements.
This year we see the release of the M10P and this time Leica have made some real improvements, a new shutter and now with a touch screen. Yes you heard right, a Leica M with a touch screen.
There are also some rumours of a new Leica M mount camera that will have a EVF. Many have asked Leica for EVF; it would not be an M but would be a useful addition to the family.