1. Enemy respawns
  * every now and again, the opponents has new units that spawn
     * indicator to show respawn is nearing
  * is it all the missing units, or a chance for each?
2. game end
  * when a side is out of pieces
3. 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
  * then the enemy respawns units, their new untis are stronger
  * support for pieces with multiple hit points
  * unit info panel
4. Visual polish
  * clean up tile borders
  * multiple square tiles to add variation
  * make the help text indicate (flash, etc.)
5. Sound effects
  * on hit
  * on piece lost
  * on piece kill
  * on opponent victory
  * if possible, a small bit of background music
6. Further visual polish
7. New units