Indeed, we can easily cite some examples. Historically-based flight sims like Warbirds have already existed for a number of years. Cyberpunk is an obvious possibility that was adopted for Nevolution's Shadows of Reality, although this title was later changed to a standalone game. Another prospect that has been mentioned is a post-apocalyptic title like Fallout Online, usually as part of a broader question about whether Black Isle Studios plans to get into the online game. As for other potential options, the list is long - vampire, wild west, Roman Empire, mythological, and many, many more.
Technology brings a number of question marks.
For example, the installed base of computers continues to increase, but with the rapid advances in technology, there is no perfect answer as to where to aim in order to be accessible to a sufficiently large segment of the overall market while also satisfying the very vocal minority at the leading edge. Maintaining static technology for a period of years is a possibility. This is what X-Box and other consoles do, but it's clearly an increasingly imperfect answer as that fixed technology ages. In addition, connectivity is an issue. Broadband connections allow more data to be transferred but exclude players without this type of access.