You know a global 1870 game seems like a neat idea…
you could have those neat ironclads and cool looking “great white fleet” warships and you could make it balanced and still be historical since nobody can really claim anybody was much stronger than another and Japan would be represented
Japan against Russia.
England indirectly against Russia ( trying to deny them a warm water port in Europe, by diplomatic blockade)
Ottomans involved against Russia
France and England vs. Germany, Austria, and Italy
US fighting a civil war then latter joining the war or not depending on which side wins ( north or south) If North wins they stay neutral and fight Spain, if South wins they fight in Europe with UK and France.
Spain might join the triple alliance ( Germany , etc).