Apparently Apple has confirmed it's coming in a bug report. Apple did the same thing with me and 10.2 back in October.Not sure that we'll see one, but that's just a guess. iOS 8 was the only version where we saw a 4th major release, and that seemed to be mostly due to Apple Music and the new Music app.
As mentioned above, apparently Apple has already confirmed the existence of a 10.4. Therefore, we should expect it.WWDC is only two months away. I wouldn't be surprised if 10.3.x is the last iOS 10 version, since work on iOS 11 should be ramping up fairly shortly.