The first thing that needs improvement is the ability to add and save complete sentence examples (English and Japanese) to folders. As of now, only single words can be saved.
Secondly, the need for formal speech conjugation lists. As of now, conjugations only cover informal speech.
And lastly, after choosing the radical when searching a kanji, there is a long, thin bar at top showing possible kanji matches. This is a major pain scrolling through to the right, trying to find the correct match. Why not have a full-screen list, categorized by number of strokes (like the Renzo Japanese app has)? This would save a LOT of time and effort while searching.
-- Update --
After using Midori for quite some time, I have to say that it isnt practical for everyday use. Seems 95% of the included Japanese definitions arent even used in everyday spoken language. They are uncommon and hard words that no one besides university professors, newspapers, or JLPT students might use. And most of the public domain example sentences are a joke. They were obviously written by egghead scholars and have little everyday practical use. Plus, many Japanese definitions of common English words are completely missing.
A note to the creator of Midori- How about releasing a Japanese/English dictionary app that only has definitions for practical usage in everyday SPOKEN situations. Dump all the other useless definitions. And stop depending on public domain sentence examples. Write your own. They are mostly terrible and completely useless. Its better to have 30,000 USEFUL entries than bragging about having 860,000 useless ones. Back to my heavy but useful Kodansha paper dictionary. Midori NOT recommended. Review reduced to one star.