If you can change a chess rule or create a chess rule, what would it be ?

Apple's already trademarked that. Definitely litigation territory.

Reinforcements:

Each player starts with two complete sets of back rank pieces, one set on the board (in their normal positions) and one set standing off the board behind their counterparts. When a normal back rank piece is taken by the opponent its counterpart from the reinforcements can be moved into play, on the piece's original square, if vacant, or the nearest appropriate (i.e correct color for bishops) back rank square, if not. (Rooks could be marked to indicate king's or queen's rook, for to ensure correct replacement square) If the required square is not vacant and the two nearest squares are equal distance from the required square then the player can choose either one of the two.

Variations:

King excluded from reinforcement rank: check and mate occur on the original king.

King included in reinforcements: original king must play out of check as usual but upon mate the king is captured like a normal piece and its reinforcement brought into play. The reinforcement king may move directly into check, or mate, when it enters the board if its square is threatened. If it survives entry onto the board the reinforcement king plays as normal until the games ends. 

Variation, for use as a method of handicapping a stronger player: In an otherwise normal game a weaker player could be allocated one of more reinforcement pieces, instead of directly handicapping the stronger player by removing pieces.

Awesome idea, really like it! :)

But it should be better played in a virtual environment, I imagine it could become confusing pretty quick OTB.

I would get rid of touch move and change it to place move as in touching a piece isn't a problem, putting it down is, but you are allowed to put it back on the same square.

Too many times I've seen people abuse this "Oh you touched the piece, you have to move it now!" when in fact it was an obvious adjust or the hand brushed against it on the way to another piece.  People try to win by not being the better player but by "gaming the system".

Shouldn't be too confusing: The board only ever has, at most, a normal set of pieces in play.

I think you should be allowed to promote to King.

If you have multiple Kings, to be deemed "checkmated", both kings must be checkmated simultaneously, which would make it very difficult to win for the player without the second king.

For example, if you have a WK on c8 (via promotion) and Black has his Queen on c7 and Bishop on d6, and White also has a King on g1, by the time Black has mated the second king, White has probably captured the Bishop or Queen, releasing the other King.

Also, if you promote to King, you are only deemed to be in "check" if BOTH kings are under attack.  At all times, at least one king must not be under attack.

This might make you think twice if you are down material whether you want to promote to Queen to try to win or King to make it VERY DIFFICULT to lose!

Lastly, let's say you only have 1 King on h8.  White just promoted to Queen with d8=Q+, and Black has a pawn on b2, and b1 is not under attack by White.  While b1=Q would be illegal, b1=K would be legal as then you have at least one king not under attack.  Moving the King from h8 to say, h7, would also be legal, of course.

No King can be captured.  You get a second king, there is no way to eliminate it, it must be checkmated along with the other one.

A pawn can promote to a queen only if at least 1 queen of the same color is on the board. It can only promote to a rook if at least 1 rook of the same color is on the board. If neither of these conditions are met (in other words, the side that is promoting the pawn has neither queens nor rooks), the pawn can only be promoted to a minor piece.

you can also promote a pawn to a king

Here you go

All pieces move the same, the board setup is the same but the object of the game instead of "capturing" the opponent's king is to capture both of the opponents rooks.  The king is now a minor piece completing the king and queen pair (like bishop and rook pairs).  The rooks are not required to keep out of attack until only one remains, then normal king safety rules apply to the final rook: a move may not be made which places the rook in check, additionally the rook may no pass through squares that are being attacked.  

Casling is a little different.  The king castles to the kingside only and the queen castles to the queenside only (optional of course).  Pawn promotion is the final difference.  Pawns can be promoted to a queen or king, but there can only be one queen per color on the board at any time, so if you already have a queen you get a king.  

Let's allow the pawn to do an en passant capture of ANY PIECE (or pawn as usual) that passes over a square it can attack. The pawn would not be limited to being on the 5th rank to do this for pieces, only pawns. Simple change, big ramifications!

In some sports it's necessary to penalize excess showboating or taunting. Unfortunately I've noticed a tendancy for those immature behaviors on this site. So let's penalize rude or disrespectful comments during and after the game with a one-half point penalty, meaning if you win and post a rude remark, your win is downgraded to a draw. If you draw and post rudeness, your draw is downgraded to a loss. If you lose and vent on your opponent, you're put in the penalty box (like hockey or water polo) for a set duration (e.g., one hour) where you are not allowed to play in any games... I know you can't legislate respectful behavior, but then again you can't just let children be children...they need some discipling and guidance.. even if they've matured into adult bodies.... That being said let me add that I, for one, would be horrified if such a rule was ever put in place in real life, but as long as we're brainstorming here, maybe this will trigger a better idea from someone on how to deal with this element of disrespect that has crept into the normally very respectful attitude chess players have traditionally displayed...

King capture ends the game. If a players moves his king into check it's not an automatic take-back

promotion - me and my brother used to play the rule that you can only promote to a piece that your opponent has taken, "winning it back"

Disguised king is my favourite idea - you could have a little sticker with a bearded king that you stick on the base of the piece before play starts!

You could also have disguised Queen.

In both cases they move just like the piece they are disguised as, until attacked. From that moment, the player can reveal the sticker and play King's or Queen's moves.

Could be a devastating "reveal"! 

Female chess players are required to play in minimal clothing...

... so as to prevent cheating :)

Then again, it sort of encourages a different form of "cheating."

Not sure if it is already a "thing", but my old high school buddy and I used to enjoy a game of what we called "losing chess". Simple object: The winner is the one to lose all his pieces first.

Rules:

1. Set-up and moves are same as for regular chess.
2. If you can take a piece you must.
3. If you can take more than one piece you pick the piece to take.
4. No concept of 'check'; the king is just a regular piece.

There are actually two interesting rules that used to be actual rules of the game but have since been removed.

One is that you can promote a pawn to a piece of the opposite colour. For instance:

 In the diagram, white (with the move) promotes to a black knight and checkmates!

The other old rule was to do with the definition of castling. Castling was originally allowed between the king (who hasn't moved) and any rook that hasn't moved, in the same line as the king. To demonstrate:

In this position, white can play 0-0-0-0-0-0-0, moving his king (which we assume hasn't moved) to e3 and the rook (which is a promoted rook that hasn't moved since promotion) to e2.

On the 40th move if your opponent still has her queen you can douse it in lighter fluid and set it on fire. 

if you manage to get 4 pawns in a row on the 7th rank, yuo win!