Author’s Excuses and clarifications
First of all I know there are a few jumps in this timeline, especially in the later half. I know that the Romans had no practical way of finding Iceland (Atlantis is Iceland). But the fact is that from the north coast of Scotland there isn't that long a way to Iceland. Also the Vikings found the country by similar accidents so at least I think this is perfectly plausible.
Although it would appear that way this timeline is not ”let the Roman Empire continue so Flavian Aetius can take over” in fact when I began this time line I had almost no idea who this guy was. It wasn't until I was knee deep in the civil war that I realized that I needed a hero to get things right. The Roman Protectorates during that era were mostly Roman in name only as several new tribes settled in those provinces regardless whether or not Flavius thought the lands were under his control.
Also regarding the minor industrial revolution. I'm not talking about full blown steam engines. It’s more of a more efficient way to make and build things. The heavy plow is a good example of a tool that revolutionizes the world without making machines. Imagine the Romans knowing the basis for steam engines but not having the metallurgy for it.
Simply put, alternate history is the exercise of looking at the past and asking “what if”? What if some major historical event had gone differently? How might the world have been changed immediately, and in the long term? Popular “what if” questions include “what if the Nazis had won WW2?” and “what if the South had won the US Civil War?”
To be more precise, alternate history generally exists as works of fiction, either in narrative (story) format or in the form of an essay or other non-narrative work, which have been created at least in part to showcase an imagined world where a change at some point in history led to events that could have happened, but did not happen in the actual past. A work of alternate history may focus on a point in the past, showing a departure from real history occurring, or it may focus on an altered world that resulted from the consequences of a departure long past.
In the vast majority of alternate history scenarios the departure from real history occurred within recorded history, but in some cases it may have been an event in prehistory, even a geological difference. The key point is that alternate histories deal with imagined or hypothetical worlds whose history is the same as our history up to some point, and then changes significantly.
Alternate history is sometimes abbreviated “AH” for short. According to some people “Alternative History” is more grammatically correct, but “Alternate History” is now well established as the common usage. Alternate history scenarios are sometimes colloquially referred to as “what-ifs”, or WIs for short. Other less common names include “counterfactual history” (used mainly by historians), “allohistory”, and “uchronia/uchronie” (used mainly in the French language).
Alternate history stories are usually considered a subgenre of science fiction, which is where you will usually find them in the book store, and there are also often alternate history “crossovers” with fantasy. It is entirely possible and in fact quite common, however, for alternate histories to contain no new science or technology, and no fantastic elements, merely depicting a world where history went differently. A great many alternate history novels and short stories have been written, and some non-narrative books. The existence of the genre goes back over a century, although it has only been in the last few decades that it has really become a significant subgenre of its own.
There is also a thriving community of alternate history fans on the web, with many amateur works of alternate history and active discussion forums. In other media, however, alternate history is relatively rare. It is found occasionally in television, film, comics, and computer games. This is probably both because developing an alternate history in such a way that it is interesting requires a level of background that comes across best with the detail of writing, and because alternate history requires a level of historical knowledge and interest that is not present in enough of the potential mass media audience to justify going to frequent effort to do it well.