How can you include language around future assets in an islamic prenuptial agreement?

Yes.

Obviously you don't know what your future assets are going to be, so that's why I like to use in my prenups the opt-in method.

That the separate property is defined as any assets or bank accounts or property in my name.

What's yours is in your name, and then any future assets that we acquire together, like jointly with both of our names on it is going to be considered the both of you.

So yeah, obviously you can't be very specific about the the assets that you have not yet acquired.