The weirdest part would probably be having castling with a non-royal king. It really helps to build position, but I don’t know what coding would need to be changed.
i request to include my "secret kings variant"

i have played two games of it with @lukegk
he won both of them but it was really fun
Which version of the rules did you play with?

The Ahab piece cannot be checked.
We were playing on the chess.com server so we had to try not to unnecessarily check the real king.
But then wouldn’t everyone just chose their queen as their secret king?
Too obvious
Also, queen trades can be forced

But then wouldn’t everyone just chose their queen as their secret king?
Too obvious
Also, queen trades can be forced
exactly

In the practice games and games they were playing, I am pretty sure they were not letting you use queen as the ahab since that was a part of the rules. I also think they did not allow rook but I can't guarantee that

We have suggested Ahab Chess in the variant servers https://www.chess.com/clubs/forum/view/ahab-chess?page=7#last_comment
They should say if it gets declined or not.
Yup. We started a game last night testing out a game with my rules, and then I guess depending on how things go after we finish this one, we could try one with his rules.
I'm wondering if we could do it such that both rules are options, where basically the Ahab rule (selecting a royal at the beginning) could be selected with either checking or capture the king, and also it would be up to the person creating the variant to select beforehand which pieces are not allowed to be chosen as royal. If that was the case, then both of our variants -- as is -- could be options to play.