I reckon since, all the characters are in the dictionary's database anyway, this shouldn't be too hard to implement. Nothing fancy, just a typical flashcard type of thing, where the user would be presented with a character and then would have to give all its Pinyin readings - and maybe perhaps one of its meanings. Then there could be a small statistic to show the user where they are as far as their Hanzi knowledge goes.
You could take this even one step further and make it into a game where the user would be presented with a Pinyin syllable and then would have to draw up as many Hanzi as they know with this particular reading in a certain period of time.
This would definitely be a feature to set the app apart from its competitors - not that there are that many...
I know there are Anki decks out there, but the beauty of having such a functionality integrated into a dictionary is that one could go right into a search that matches words containing a certain character instead of having to switch between apps.
Also, how does a "Hanzi of the day" option in the settings sound to you?
Well, apart from that your app is pretty much perfect. Keep up the good work!
- Miriam