Looks Great!
Ahab Chess

Love the Bible story XD!
That sounds like a really cool variant! Actually, maybe it could even be an enabling option along with Crazyhouse, Seirawan Chess, Giveaway, etc.

That's what I was thinking, too.
Also, castling would need to be thought about. I think it would be fine to just have castling work like it normally does, even though the king isn't royal. Another option would be to disable castling altogether.

sounds really cool.
one thing i would suggest is making you not be able to chose your queen as royal, because it is almost impossible to checkmate a royal queen, so you would almost always chose your queen. all the other pieces could be royal, as it is possible to trap them easily.

That would probably be smart. You generally don’t lose a queen unless you blunder it or trade it, so it might not work out having it as a royal.

That's what I was thinking, too.
Also, castling would need to be thought about. I think it would be fine to just have castling work like it normally does, even though the king isn't royal. Another option would be to disable castling altogether.
I would say just do castling as normal, don't castle the actual disguised king, just the "fake" king.

What if you COULD select the king as well? This would allow for castling to actually be effective and it would still allow the element of mystery.
Also, this needs to be added, it's one of the best ideas i've ever seen! @Max_Wolfe are you seeing this? lol

@Peet, I am open to that idea. Personally, I like the idea of already knowing the king is a decoy. And especially if we’ll also be making it so you can’t make your queen be Ahab, I think it makes sense to prevent your king from being royal, too. But if it would be better to give people the option of selecting the king, that would probably be okay.

That would make sense... the queen would be too powerful and it would be weird for the king to be considered when the queen isn't.

Thanks for the input, though! I’m really glad this idea is getting so much interest and positive feedback.

If you castle the fake king wouldn't the opponent then know its the fake king
??? The whole point of this is you choose a piece to be the king, the opponent already knows its not the king. The reason I suggest this is even though it doesn't protect the king, it could bring a better position, for example, it can bring the rook into the action.
This is a pretty goofy idea for a variant, but I think it would create some really interesting gameplay and add an extra challenge.
The name "Ahab Chess" comes from a wicked king of Israel in the Bible (named Ahab, of course). In 1 Kings 22:29-38, it tells of an event when Ahab went to war with another country, and he disguised himself as a regular soldier. When the enemy army realized that the "king" up in the chariot was the king of Judah, not Israel, they were confused and didn't know what to do. However, Ahab was struck at random, and ended up dying.
Now that I've finished that little Bible history lesson, perhaps you see where this is going.
My idea is that the board would have a standard chess setup, and at the beginning of the game, each player selects any one of his pieces (even a pawn if he really wants to) to be his royal piece. The only piece that cannot be royal is the king (in the story, the enemy realized that the king was just a decoy). [EDIT: The Queen would be prevented from being royal as well, because of how difficult it would be to capture a royal queen. This suggestion came from @noahfavelo in the comments below.] Neither player would be able to know which piece has been selected as a royal. This would be a capture-the-royal variant, so you can knowingly put yourself in "check" pretending your piece isn't the royal, taking the risk of possibly losing, or just throwing your opponent off.
What do you think? Is this another dumb idea of mine? XD or would this be good?