1. Power ups
  * a chance of having power ups spawn on an random unoccupied square when a unit dies
    * +health, +attack, +speed, +jump, (pawn only) remove "attack_only", spawn a new (random?) piece, change movement type
2. Visual polish
  * multiple square tiles to add variation
  * make the help text indicate (flash, etc.)
  * visual indications when damage is done but a unit isn't killed
  * visual indication when a unit is stronger (eg. a little skull or star or something)
3. Sound effects
  * on hit, especially when the unit isn't killed
  * if possible, a small bit of background music
4. New units
5. Further visual polish
  * animate the piece making the attack, and returning to it's spot

Bugs of note:

 * n/a