The tasks are arranged in the approximate order I am planning to tackle them.

+ - alpha code, please test and report any major problems
* - beta code, should be working, report any problems you find as bugs

Basic functionality
 + Open file
 + Open URL
 + Start
 + Stop
 + Pause
 + Quit
 + Full screen
 + Context menu
 + Shortcut keys
 + Main window title
 + Volume
 + Mute
 + Seek
 + Autosize
 + Status
 + Recent files
 + Maintain Aspect and Force Aspect options
 + Persist options between sessions
 + Maintain video size when docking states change
 + Enable and disable actions according to state

Additional controls
 + Zoom
 + Frame drop
 + Contrast
 + Brightness
 + Hue
 + Saturation
 + Popup sliders
 + Display mplayer text output
   Tooltips when dragging a slider
   Audio delay
   Equalizer

Subtitles
 + Load from file
 + Load from URL
 + Autoload
 + Unload
 + Delay
 + Position
 + Visibility

Configuration dialog
 + Tree like interface
 + GUI-related options
 + MPlayer executable path
 + MPlayer additional options
 + Drivers and codecs
 + Cache size
   Filters
   Fine-tuning wizard

+ Drag and grop

+ Error handling

Playlist
   Path or URL
   Type
   Length
   Formats
   Bitrates
   Settings

Per file options

Playing from kioslaves

Documentation

DVD and VCD support
 + Playback using dvd:// urls
   Menus
   Chapters
   Titles
   Audio language
   Subtitles by id or language
   Devices
   Eject

Integration
 + MIME file types
   KPart

Mencoder support

Advanced options (may require patching mplayer)
   DVD options
   VCD options
   CD options
   TV options
   Just about every option from mplayer(1)

Config file handling
   Read
   Write

Maybe
   Plugins
   Skins
