What will be the next version of Android OS?

It is too early to speculate on this aspect as Google has not yet come up with any update on this. Right now, Google is busy releasing MRs - the Maintenance Releases for its Lollipop update.
We will need to wait for the complete roll out of the Lollipop update before any speculation can begin on the next release.
Well, one thing is sure - the next version will be named with letter M - anything from Milkshake, Mint or some other dessert beginning with letter M.

Google has not announced about this yet. Latest version Android v5 Lollipop is just released and updates of this OS are still coming with minor or major bug fixes. So we can say that Lollipop is not yet fully successfully released. So its too early to say about next OS.

But we can say that next OS will me more tasty and yummy than Lollipop like Maggi, Mayonese, Momos, Muffins or Mango Lassi.

An update here- The next version of Android may be announced at Google I/O event. Guessing game has begun for the Android M. Some possible names doing the round include Muffin, Merringue and Mud Pie.
Possible release date is being speculated as October or November , 2015.
Possible features expected are -
1. Improved notifications - Notifications are likely to be unified across all platforms. Major changes are expected in the way notifications are handled.
2. Smart Home - It will be an extremely important part of Android system.
3. Security - After seeing "Find My Phone" feature enabled for Chrome, you can expect better security features in Android M.
Please note that these are too early days to come to any conclusion concerning the features of the new update.