Are you aiming for it to be "realistic" or will there be indian shamans vs western technology?

Think Will Smiths movie "Wild West" and the giant mechanical spider.

  Kinda aiming for alternate history, and yea using semi-realism.  Kinda a mix of "american" history, movie and dime novel mythology. Maybe some scenarios would have what your asking about MichaelCook but I definitely would not be using magic like Tasunke is suggesting.  Shamans will be more like a political figures, with morale, loyalty and combat modifiers for their tribe.

  I would like to place "my" scenario in 1800-1900 but it could start anywhere from say(and being partly arbitrary)1600 up.  I would like to have wagon trains, cattle drives, military and civilian 'conflict'(thinking of the various wars and land disputes between US and Mexico, US and Canada, US vs native americans, Johnson County War, Lincoln County War, Kit Carson, Jim Bridger, the Alamo, cowtowns and saloons, railroad building.

  Would be a combination of empire building/management, hero development, with scenarios involving Wagon Trains trying to get through "indian" territories, taming a boom town, hunting outlaws and flaunting the law, founding a town, becoming a large rancher, civil war, fighting back against the white man's incroachment(some indian perspectives), counting coop, go on a spirit quest(for shamans), trade and barter, wars between tribes, etc.

  Also "this is just an idea at the moment" and not a definitive work in progress .  I only have one little gimp2 created "cactus" picture, a map, and this rough outline.  I would be pleased as punch if anyone thought enough of this to do something with it.  This is an 'open source' idea.  If anyone wishes to take this all the way and make something tangible I would be pleased.  My skills are so limited that I don't even make a good 'beta tester'.  I shall attempt to write some stories or scenarios for this.

Have the Mechanized Wild West, with Steam Tanks and Mechanical and/or Biomech engineered steeds, with a Shamanistic Indian Tribal outlook wielding elder magics and RIding magical beasts.

Then aliens invade ... with their plasma guns and teleportation gates, and their UFO's. Indians can use their magics to shoot the UFOs out of the skies, meanwhile the Westerners have their Giant Spider Fortresses with arrays of machine guns and cannons, and perhaps even Tesla Coils.

At western home-base, a Tesla Coil can power an entire city, or it can power 5 level 1 settlements, or it can power a Giant Spider Fortress. A mechanical City with two tesla coils can construct and maintain one Giant Spider Fortress.

Giant Spider Fortresses can eat into the country side to harvest raw resources, and can also build new Bio-Mech steeds, and produce more Tesla Guns. (can build/train units) ... A Giant Spider Fortress is in many ways like a Mobile City with lots of Offensive Fortifications. (wild wild west)

The aliens' ground attack crews generally involve a Wierding Tree station (glowing city made of alien trees), with a Teleportation gate to gate in Units, and to Teleport to other gates. Cities resources and trade can be connected by Trade-Gates as well. They attack with plasma-rays, Sun-Scorcher beams, and their super-intelligent and ferocious hell hounds they bred for war.

Its a battle Royal Between the Three Main Divisions, however within each division their are various factions vying for power. Amongst the Westerners, its pioneers from the Varoius colonies, although the Westerners are perhaps the most united (at least when the Government Issued Death Robots arrive). The Aliens have ancient Royal houses that constantly battle each other for honor. While the Native Americans are peacable with each other, they hardly ever work together, and usually try to fight threats off alone, with their personal Ancestral Knowledge. Its a matter of Pride.

Who will survive? Who will be enslaved? Who will experiment and torture who? (the last one is for the aliens)

The aliens could have an Experimental Labs building, and an Abduction facility that uses special, fast, stealthy non-combat UFOs for abucting experiment/Test subjects ... so that they can develop anti-personell weaponry.

The most "powerful" weapons are the Magics of the Natives, while the Mass Producable Machinery of the Westerners makes them another powerful threat. In comparison, the Aliens have to hide in the shadows, using teleportation, hell hounds, and abduction UFOs in order to bide their time and complete their research (because they have a Larger and more powerful Tech Tree). Once the Aliens have re-united with their mother-ship however ... all but the most powerful Terran armies would be able to fight off the Vast and Powerful Alien Menace.

In the mean-time ... the politics of Man vs Man landgrabbing continue.