How did I miss this?
Is this what I think it is?

In the settings menu....
Can we enter the code for entirely new play and hit save and it will add that play to the game?

Please tell me that is what that button is. And if it is how come it hasn't been mentioned. For some one that wants to greatly expand the plays in the game, that's a big deal!

it is.

You can also just edit the Plays.xml to add more plays

Could we at some point have the ability to do this using a diagram where we could point click and drag players around and give them routes any way we want? It's a little difficult to be doing this be entering programming code.

A diagram is not in the vards for the foreseeable future. But maybe at some point sown the line.

The xml is fairly easy to edit though. Francua posted a guide on the modding forum.

Unless I'm missing something (correct me if I'm wrong), it appears from looking at that guide that only routes can be modified, not the actual formations. Modifying in a diagram might allow us to re-create any play ever run in the history of game along with anything else we might come up with ourselves in only a few minutes. I wouldn't expect it to added until a future version though, perhaps DDPFS20.