Its hard to say what I expect to see in IOS 10.3. I am sure there will be security improvements, bug fixes, as well as performance improvements. But in terms of features its hard t say what I expect.
On the other hand its pretty easy to say what I would like to see:
1. Safari Notifications, with syncing of notifications through iCloud (Would love to have this on IOS).
2. Improvements to Photos, addition of Sharpen, Definition, Noise Reduction, Vignette, White Balance and Levels Tools would be awesome.
3. RAW editing support for Photos. Also RAW support for the native IOS Camera app would be awesome.
4. Open up the number of topics/channels one can have as favorites in the News App. (I am maxed out and keep finding more I want to add.)
Other Things
5. Dark Mode would be cool to see, but I think implementation needs to be more wide spread that what its like on macOS.
6. More full screen effects for Messages. More Bubble effects for Messages.
7. Improvements to TV App. More providers for Single Sign-on. (Would love FIOS).
8. Also, I dont see much need for split screen multi tasking on iPhone, however I would love to See Picture-in-Picture on iPhone (Especially for iPhone 6 Plus, iPhone 6s Plus, iPhone 7 Plus devices).
But again, who knows what will happen, these are just somethings I would love to see. Also I know I am kind of barking up a tree with such a long list, were lucky to get 1-2 real new features with a X.X update.
Edit: Forgot to say 9. More Emojis (LOLS), as long as they are support the newest release of Unicode that is cool.