Android 6.0 Marshmallow was first unveiled in May, this year, and released officially in October. While it comes with a new letter (M), it didn't bring any significant changes over the 5.1.1 Lollipop. One of the biggest improvement is the new permissions architecture, which, according to our sources, is going to be improved in 6.1.
One of our most trusted sources told us that Google is planning to release the Android 6.1 Marshmallow in June, 2016, and it will also come with the long-awaited split-screen support, along with a better way to deal with permissions. Of course you should take this with a grain of salt, but we've had plenty of leaks that turned out to be true from this source, so it's most likely the real deal.
Android 6.1 should be another relatively minor update over the 6.0 and it will most likely come with the same name. We don't have much information right now about it, as the release date is pretty far, but, according to our source, Android 6.1 Marshmallow isn't expected to come with any new features, except the ones aforementioned.
Like I've stated previously, the current permissions system seems pretty much a mess to me. While it's clearly better than before, it can be really annoying. One good example is that if you grant any app the permission to draw over other apps, you can't change any other permission, to any app. I understand that this might be due to security concerns, as one app that has the permissions to draw over other apps could change the text in the permissions screen and trick you into giving it the wrong permissions. But I'm sure that Google can find a better solution to this, as it becomes really annoying.
Google is expected to focus on the split-screen support and the improved permissions system, along with bug fixes in the next version of Android. We hope to find out more information about it in the near future and we will keep you posted.