I am considering something more historical in the near future for a new campaign. I really like history, and tend to stick to it fairly closely when I incorporate it into a game, but I like alternate-history and games that deal with time travel. There are also times I don't want research to get in the way of a fun idea. The following approaches are what I am contemplating using. I haven't tested them out yet. By the time I run them, these ideas will probably change a bit. They are seeds.
This is an approach to alternate history that I want to try. Basically I'd pick a time and place that interests me, for example Rome or the Silk Road during the reign of Ghengis Khan. I pick a year and write down everything I know about that time and place, including geography. I am not allowed to do any research to fill in the blanks. The blanks I have to fill in myself, and they must be interesting as well as vaguely plausible. I can plug anything that interests me into these gaps so long as I can make them believable. I can include supernatural elements also if I want, but most likely won't. This applies to everything: the history leading up to the present moment, the people in power, the technology, the institutions, places, etc.
So if I am doing Rome in 70 AD and only have a vague notion of what lies south east across the Red Sea, I can put anything there I think makes sense. It could be something based on what I imagine resides in that region. Perhaps I am vaguely aware of the Kingdom of Himyar, but don't really have firm grounding in what it was, so I come up with something that I believe would be interesting: an early christian kingdom founded by a forgotten disciple that is sitting on a wealth of frankincense and myrrh. That is far from the truth, but it is the sort of detail that might emerge from this process. I can take it further and elaborate (they could subscribe to a strain of Christianity that the player's never encountered before, where the eucharist is literally the blood of sacrificial victims). This could be a cannibalistic sect responsible for all those nasty rumors about the religion in Rome. Maybe their rituals even work, and they have a direct line to Jesus. I don't know. Whatever shakes things up. I could apply this to institutions as well, creating all kinds of social groups and organizations in the capital to replace my ignorance.
The key is to use gaps in your knowledge to reshape the past and its geography.
TIME TRAVELERS OF IMPENDING DOOM
This is an idea for a time travel campaign. The premise is that in the present day or near future (whichever the campaign is set in), a group of researchers and historians discover the secret of time travel and use it to actively alter the past for their own nefarious purposes. However the campaign doesn't begin as a time travel campaign, it starts as something else (FBI, Mafia, Super Hero, etc). Slowly the players become aware that something isn't right because every day I roll on a altered timeline table. This would be a deep table with several areas: politics, ecology, entertainment, technology, player characters, events, etc. It would also have a range of interesting outcomes that range from the mundane to the ridiculous: the automobile was never invented, aliens!, government replaced by monarchy, etc. The rolls would produce one or two major changes to the present every day. So they players might wake up to see an interview with president Bill Murray on the television talking about his recent visit to the speak before the grand council of the Berkshire Empire, America's closest ally. They would know that neither of these facts were the case the previous day. The next day something would happen. It might even directly affect the life of a player character. There can also be supernatural or weird results on the table. As things keep getting more chaotic, the world becomes more and more dangerous as marauding street armies and dinosaurs threaten the stability of the world. The players can follow the rabbit trail to the heart of the mystery, eventually going into the past themselves to fix each change and stop the bad guys, or they can contend with the emerging threats in the new reality.
The key for making this work I think, is going to be noting what change to history is responsible for each table result so there are definable problems the players can tackle. This also has the effect that the longer they delay, the harder the clean up effort is. I like this because I enjoy games where there is that kind of building pressure.