This is one of those ideas that could be brilliant with the right group, but could just as easily all flat. The central idea is that the characters are sent back in time from a future alien invasion in order to prepare humanity for when the aliens come. The emphasis of the campaign would be on the characters trying to get the planet united, and using their knowledge of the future and technology to prepare humanity to fight off the coming invasion.
Before the game starts the group should work out a timeline for the setting, with technology developments, as well as social and political events. This will provide them with a lot of foreknowledge when they go back in time.
Playing the Invasion
You can start the game during the alien invasion, possibly even with different characters from the ones that go back in time. It would be a good way of making the stakes clear… And for the last session of the intro, you could have a desperate defence of the particle accelerator as the characters try to finish their transport…
Long Term Planning
The Essence of this campaign lies in the fact that the characters have to think long term. They are trying to shape the entire society of the planet, not a simple task. They do have some advantages, but they are not massively overpowering, so they are going to have to plan ahead. In this campaign, the GM should be responsive to the players, not the other way around, most of the time.
The Butterfly Effect
While the characters will have a very good knowledge of the future (since they will bring historical records with them), over time, as they are active, this will become less and less accurate.
While the players will try to anticipate the results of their actions and changes, the GM should make sure that things don’t always go the way the characters expect.
The Mechanics of Time Travel
Time travel in this campaign is used as a device to get the characters into the past, and I do not really see it playing a big part in the campaign. Players being players, there is always a chance that they will figure out a way to make more use of it.
Whenever somebody travels back in time, a new timeline is formed, completely independent of the original one. What happens to the original timeline, whether it ceases to exist, or still exists in some form is not really relevant, since there is no interaction between the two timelines.
What this means that you cannot really alter your own personal past. If you kill your grandfather, it just means that you are now a murderer, and that there will not be another person like you in the new timeline.
In this setting, while time travel is possible, it is not simple. It requires a large, well designed and calibrated installation, that would take years to build (and that is assuming that adequate financing, land and government permissions are available).
Most of the stories in the campaign would revolve around the actions of the characters, as they try to set the world right. Here are some ideas.
If the characters start simply telling people about the coming alien invasion, they are not going to get much support. A demonstration of their advanced technology might sway some, but there is always the risk of the technology simply being confiscated by the government of the country they are in, and the characters getting brought in for questioning. How are they going to get support?
Creating a Conspiracy
It is entirely possible that the characters decide to hide the truth. Perhaps they think that nobody will believe them, or they are afraid of a world-wide panic if people did… Either way, they choose to work underground, hiding their true plans and intentions. Do they establish a cover as a company, or some sort of religious organisation, or something else entirely?
The characters know that some sort of large-scale disaster is coming. Tornado, Tsunami, Earthquake, that type of thing… What are they going to do about it? Will they try to warn people, or prevent it in some way? Or perhaps simply get themselves in position to take advantage of it?
Every now and then, it might be good to focus on what the characters do for their personal agendas. If somebody’s mother or wife died of cancer on the original timeline, are they going to try to prevent that, perhaps get her to quit smoking in this one, or try to get her to get diagnosed early enough? Or are they going to spend a lot more time and energy in those areas of research? All of the characters would have lost people they loved in the invasion. Are they going to try to see them? Even though their loved ones will most likely not recognise them.
It is entirely likely that the characters themselves are on this timeline as children. Are the characters going to try to avoid the alternate versions of themselves, or will they try to warn them about the things they might regret later?
This campaign, by its very nature will delve into politics. The players may want to unify the world to prepare it for the coming threat, but how will they do it, will they try to engineer a single country taking over everything, or will they set themselves up as puppet-masters, allowing the disparate countries to believe that they are not acting in unison? Maybe they will even try to get countries co-operating out of their own interests.
Another approach that would help is to advance the technology of the world. The characters have a definite headstart. Of course, this will involve a lot of the politics mentioned above, as well as dealing with other companies, nations and entities.
Here are some ideas for spins on the basic campaign outlined above, or ways to spice up the campaign once it has already started.
Other Time travellers
There is always a chance that the characters are not the only time travellers. Perhaps they find out that there are others that have arrived before them, or run into versions of themselves from a future in which their meddling has made things even worse. Or, the aliens send somebody back, since they were so successful, and the invaders want to stop them. This alternative would work well in an already running game.
In this alternative, the alien fleet has sent advance scouts, which arrived decades or centuries before the main fleet, and infiltrated humanity in order to make sure that they are not ready when the main fleet arrives. This would work well if the players want a direct enemy, and could be used to run a conspiracy investigation campaign, as the characters try to figure out who the infiltrators are.
They might even know some of the infiltrators, from their own time, but what are they going to do about it? Nobody would believe them if they started telling people that the CEO of a major corporation is an alien spy!
What if the characters go back in time much further? Ancient Rome maybe, or even into prehistory on the African Savannah. Do they establish themselves as gods, or do they just try to lead their tribe? This also causes a problem for the characters, since they will need some way of ensuring their plans continue even after they are gone. Do they try to establish some sort of lineage or cult to pass on their teaching? This might also work as a generational game, with the players playing different characters through the ages.
For this, you can set the game in a Superhero universe. If superheroes already exist in the world, and are known, the characters would have a much easier time convincing people that the threat is real.
For another take on it, perhaps the characters use their advanced technology to establish themselves as Superheroes in the new timeline?