Attack alliance player's unit

I think it’s really needed from time to time.
some players’ missing units’ but too far away from his base(go back to homebase too long)
or building block.

And Team kill onager etc…
and. Mainly it is natural.

And any aspect it may help custom scenario more diverse

This can already be done if you click nonlocked teams when hosting a game.

Locked teams games are fine. You are meant to be loyal and honorable in such a game. The guy who team kills I’m sure would be punished in some way. Unless, it’s a diplo game, then it’s perfectly fine.