I am a new kdenlive user (and openshot user, too). Before and now I am working on commercial windows video editors (Sony Vegas Pro) and I propose this list priorized by my own use. Of course, you must evaluate difficult to implement some ones.
1. Adding effects by creating an alpha channel over a videoclip based on luma values (color values on a color channel). This effect gives an alpha channel: a pixel was as transparent value as you define a R,G or B value (between 0-255). This effect could be inverted (0 for transparent and 255 for opacity).
2. All effects could be keyframables on all parameters. Priority on this effects: speed, rotate, etc.
3. A different behaiour of rotate effect. Not only a fixed rotation or an incremental rotation. I think is better implement a keyframable rotate effect which fix some keyframes about final rotation. Maybe interesting define new parameters to stablish origin coordinates for rotate, not only screen center.
4. Title editor must implement rotation and other parameters (type letter, size, etc) keyframables to get animable titles.
5. On clip monitor it is better to see transparent background on a customizable colour, not only black. If foreground is black too (example on titles), all screen is black.
6. Add on tree project an option to change a clip to other one different.
7. Add 5.1 sound edit and render (so, it is necesary to implement a new audio pan effect keyframable).
8. On title editor or video effect, add a shadow option.
9. Add on menu a new option for import new mask files (*.png files).
10. Write an user manual guide similar writed by openshot (kdenlive has an incomplete user guide).
11. On timeline, show keyframes on a track effect about track opacity and track sound volume, similar to Pitivi implemtation.
12. Add new profiles to work and render on 1920x1080 and 50p projects. And an export option to create mkv files.
13. New option to select some clips on tree project and put to timeline between two layers stablishing an default transition: defining a fixed type or random type and duration time).
14. Autosave option configurable (define time intervals between autosaves in minutes and differents on user saves). For example, a project is saved as sample.kdenlive, but autosaves as sample-auto1.keenlive; sample-auto2.kdenlive. Application must save a determined autocopies (for example 4 copies).