There won't be a successor to the M, at least no meaningful one. Variations on its theme, maybe an incremental sensor improvement here and a feature enhancement there, but no realistic successor as the M240/246 is to the M9/MM/M8. In the same way that Leica's film M's have reached their endpoint, the digital M product curve is matured and done. The future line for development is the Q. Now fixed-lens to test the waters, soon to have an interchangeable mount, and then a line of Q-mount AF/MF lenses.
Think on it a moment (if you haven't already but I'm sure you have). A camera the size of your film M's with comparable haptics, with comparably small and fast lenses, a truly excellent EVF, AF or MF (on demand, you choose), digital imaging for all purposes equivalent or better than that of Oly/Fuji/Sony bodies. Whew. Get on it Leica, bravo.