If you want something historically accurate, you have to realize that there were several systems.
In the Americas the USA actively guaranteed to intervene if the Europeans tried to recolonize (looking at you Spain and South America).
In Europe, you had the concert of Europe, went something like this.
The Seventh Coalition (1815) turned into the Quadruple alliance:
Added
France in 1818, thus creating the
Quintuple Alliance .
These states could decide everything in Europe and would create the concert of Europe to keep the peace.
In 1826 this failed due to disagreements about the Greek revolt.
The alliance split more and more into separate factions. The more conservative members joined the Holy alliance (1815-1854, 1871-1880):
Starting with the 1848 revolution, France left the alliance, followed complete dissolution of the alliance with the Crimean war.
New alliances formed, like France and Britain teaming up against Russia during the Crimean war, Austria and Prussia against Denmark in the Second Schleswig War.
A big change occurred when Italy became united with the help of France and Prussia.
In 1882 Austria-Hungary, Germany and Italy united in the Triple Alliance (1882-1914), which led to France and Russia creating their own alliance (1892-1917).
The United Kingdom also allied France, creating the Entente Cordiale. In 1907 France, Russia and the United Kingdom formed the Triple Entente. The stage was set for WWI.
I won't go into detail about post WWI great power influence and system (basically just the victors of WWI controlling everything).
Throughout the 19th century Europe, Africa and Asia were controlled by the members of the
Quintuple Alliance. They could straight out invade a country whenever they felt like it, as they did in Italy, Spain, Belgium, the Ottoman Empire, Hungary and other places. But when the members did not agree on a course of action, it could mean war or an intervention at the peace conference, as they did 1878 in with the
Treaty of San Stefano.
Conquering a whole continent was fine, redrawing the map of Europe was also fine. As long as they agreed it was fine, which usually meant that they all had to benefit. In game, I think this would translate into some sort of complex trade system where you can annex Morocco, but Germany gets a colony for example.
If no agreement could be reached, you could force your way through and risk war or maintain the status quo.
Perhaps it can be represented with a comparison of relative powers. When one member gets too powerful, the others will oppose him more, unless they are closely allied.