No Retreat Chess

Pretty simple change-no piece may ever move backwards.

Intended consequences on the game:
-Should create more cautious, army vs army formation play, similar to Chaturanga, while retaining the potential dynamism present in modern chess.

-Achieving this with a simple rules adjustment is also clean and easy to learn.

-Most moves will be cautious but moves can also be dynamic- however dynamic moves now come with an opportunity cost.

-Achieves a feel of "battlefield honor" and "Impending doom" similar to shogi.

Side effects:

-Sideways pieces (Rook, King, Queen) have extra layers of strategic meaning. They can hold a front without advancing. This also makes them more "powerful" pieces as a result.

-By the same principle, the Knight is now slightly more powerful than the Bishop (rather than slightly less), as it can achieve more sideways movement with less forwards movement.

-2 Square pawn movements now may be considered slightly less powerful, and castling slightly more powerful.

-Games may take longer in opening, but, unlike Chaturanga, they may also speed up once a worthwhile advantage is gained.

-Structures will slowly crumble as you play, and there isn't much you can do to prevent that by retreating pieces- you must form a new structure somewhere ahead. 

-may need a ruling on stalemate/victory when the all pieces get passed.

-Pawn promotion doesn't mean much, and may need a rule change similar to checkers or chaturanga "Queen promotion" (I'm actually a little partial to making promoted pawns full strength Knights, but what do you think?)

I figured out a solution to pieces getting to an unuseable end- Fill all 8 squares of the opponent's backline with your pieces is another way to win.