* LevelGen: when making corridors, avoid putting two doors next to each
  other to bridge a two-square wall.  In fact, special cases for two
  and three square walls might be desirable.  Two-square can look like
  .+ or +., and three-square can look like +.+ or .+.
* In the far future, some kind of "before-a-door" feature could be
  added, for example a welcome mat :)