This is an idea I have had for the longest time so I figured I'd give it a(second) shot.
Ahem.*Dramatic pause* In the year 1872 Thomas Edison invents the Anti-graviton Dynamo(Name too silly? I was going for pulpy but well....) this device allows for the manipulation of gravity, and with it man could enter space. The planets of our solar system are not as we remember them as most all of them are inhabitable. Venus is a world covered in rainforests and marshes that are filled with wide arrays of wildlife including the intelligent Venusian, large reptilian people that have been exploited for cheap labor by the human colonists. Mars is a world of rolling red planes with ruins of a lost Martian civilization dotting the landscape. The moon is, or rather was home to the great Lunar race, a species of xenophobic insectoid creatures who upon discovery of man waged a bloody war against us. Now only one small city remains of the Lunar race, carefully watched over by armed garrison lest they try to strike again.
The Year is now 1888, Space travel has become cheap enough that anyone that can afford a hull can afford a ship. Free merchants sell their wares all across the system. Pirates Stalk said merchants and make off with the booty hopefully avoiding patrols. Some people just want to explore the solar system as The solar system is huge and there are innumerable places to explore.
One of the first things humankind found when they first landed on Mars were the ruins of a lost great civilization. The Lunar race knew of the Lost Martian civilization but go near catatonic when asked about their fall, merely saying that they have been gone for centuries and then immediately change the subject. All that remained were a few scattered buildings and scraps of technology. This was soon to change however. In the Terran fall of 1888, A large group of strange spaceships were discovered at the edge of the system near Charon. The ships are what remains of a Martian expedition in force to another star that was dispatched centuries ago, that is only just returning home. They found both their and the Lunar civilizations in tatters, with only the humans having become a space power. They put two and two together to get nine and declared that the humans must have destroyed their homes and vow revenge. While they have been depleted over the centuries the Martian fleet still outnumbers any three Space navies in the system and is more advanced to boot.
The Pc's are any group of people from merchants, pirates, soldiers, scientists etc. who made first contact with the Martians either intentionally or accidentally and are caught up in the middle of the ensuing conflict between the Martians and Mankind.
The technology of humanity is what would be expected of the late 1880's with notable exceptions. First is The Anti-graviton Dynamo. This device creates what amount to pushing and pulling through gravity allowing vessels to move.You want the science behind it? There is none I'm throwing science out the window for this Rp no matter how much it hurts.
Next are Heat rays. Heat rays are a throw away name for most Directed energy weapons. The only components that Humans cannot produce is the focusing crystal. The Lunar race used the Crystals that grew in the massive caverns under the moon but these were underpowered compared to Martian ones. The Martians used Jovian crystals which I am handwaving as being better. When fired, a heat ray produces a lance of visible energy that burns through most metals with ease depending on the size of the weapon.
Then there are the more Steampunk elements as mentioned in the thread. Things like a steam powered space elevator and robot arms. General things like that. Dynamo's caused the invention of the bullet train a century early.
Those are the main tech differences really.
As mentioned above the planets of our system are nowhere near what they are in our world. Mercury is still mostly inhospitable and Pluto is still a cold ball of uselessness however. Venus is as it is mentioned above as is Mars. Jupiter is no longer a gas giant(Not my Idea but I kinda like it) but a massive world of Mountains, plateaus, and steppes with most life living in the Valleys. Saturn is Also no longer a gas giant and is a world as if it was made from the Fever dreams of H.R Giger, H.P Lovecraft, and that dude who made the drooping clock painting. Uranus is um....Haven't gotten there yet. And finally Neptune is an icy world but underneath said ice are massive oceans filled with a plethora of life.
(Beware walls of text for the beginning of this)
The Nations of this world are pretty much unchanged from their real life counterparts sans a few changes. Czar Alexander II was never assassinated and because of this the Russian people gained a constitution and other civil liberties they previously lacked. With the situation at home relatively secure now, Alexander wanted to make the name of Russia great again and Commissioned a young inventor named Nikola Tesla the paltry some of 10 million rubles to develop a Dynamo for the Russian Empire making them the third nation to make claims in space(British empire and the Us being the other 2) The Russians expanded greatly using the resources from Venus and Mars to begin modernizing the Motherland making it a Nation to rival the British Empire.
The United States Had been war weary during the opening stages of the Great Moon War, but once the Lunar race attacked human population centers in the infamous London and Paris raids, the American people once again found their resolve to do battle. Under the motto of "Keeping Earth safe for Americans" Presidential candidate Jeremiah Steward won in a landslide vote. President Steward broke America's isolationist policy and began to aggressively colonize the Sol system as an act to "keep Earth safe" This lead to the United States becoming one of the three major human powers of the day.
The British Empire did what it did best and Imperialized the greatest swathes of the inner worlds, owning an entire half of Mars and a quarter of Venus. They are the second power to enter space, but are the largest by far.
Imperial Germany, France, and Japan make up the secondary powers as they were a little slow to the space party and as such have less land claimed.