If they don't backtrack on the camera in some respects like they did with the M8, I might get the M9. The M7 was a good device overall, but I had the blue/purple tint or haze issue in low light after a few months (something to do with the camera module overheating, IIRC). The camera quality wasn't bad otherwise as some people made it out to be - macro shots with it in particular are fairly excellent - but the One was clearly outclassed first by the LG G2, then the G3, GS5, and Note 4 (and the iPhone 6 and 6 Plus).
The M8 improved on the battery life of the M7 and made the excellent speakers even better, but the camera isn't as good (especially because they removed OIS) as the Lumia 1520 or Icon which I have now. I'll certainly consider getting the M9 if it has a better camera than the Note 4.