Are questions about fantasy sports on topic? This is a huge market and a huge aspect of fandom.
Are they on topic?
As an avid fantasy sports player, the problem allowing that topic will be the proliferation of questions like:
"Who do I start this week?"
"Should I draft Pujols over Cabrera this year?"
"Should I make this trade?"
The bulk of these questions will be too localized or have minimal longevity. While the StackExchange platform would make a nice implementation of Fantasy Sports questions and answers of that ilk, it doesn't seem appropriate for this site (or gaming or webapps)
We must also be driven by the expert questions. What are the questions that will attract experts?
I think this is an easy yes. The primary reason why is because experts in whatever sport the fantasy question is about will likely be attracted to something that quantifies their experience. At the very least, they will have the knowledge and expertise to answer questions., and since they're on a QA site probably also have the willingness to answer questions.
Willing and able experts means (imho) we should allow it.
If there is a fantasytag, this would attract (as @corsiKa intimates) very knowledgeable contributors, while offering those not interested a way to ignore their fantasy specific content.
Indeed, most users that will have fantasy knowledge will be most welcome here to answer sport related questions, but I think that the online portion of the game is out-of-topic.
I think it's more suitable in gaming.se than here.
This also leaves all virtual worlds and online games out of this site. (which I think is correct)
@Zack is on to something when he says, "the most common questions [regarding fantasy sports] is 'who should I start'."
For Sports SE, however, I envision more "Who should I start and why?" to align with the question being "asked in an objective manner that provides sufficient context." Also, this format promotes semantics and strategy rather than "situational" and "opinionated" tones.
For example, I have two fantasy players I am considering to start.
If I am looking for more of a consistent source of points, I'll go with player B. If I am going with the possibility of more points, I'll go with player A. Of course, there are other factors to be considered and that can be provided. This example lays out potential strategies as to why someone should start player A or B.