With a Minimum of two main caliber turrets on a ship, its almost impossible to create a more modern Monitor design. So smaller hulls to be able to make them can be added, and the limit of two main caliber turrets could be lowered on these hulls to make it possible to make ships like the Arkansas-class, Lord Clive-class, or the Khasan. As of now I have no real Ideas for new weapons for them except for the Fixed 18in gun on added to the back of the Lord Clive and General Wolfe, But there's no good way to implement that.