adding in Aliens would decisively turn it into a Sci-fi/Fantasy game, which is a whole new flavor

Exactly.

I think the intent of my original question was how make the flavor that I'm familiar with work with the flavor my player would want to introduce.

Okay . . . so I want to run a DF game for my group. I'm still in the very early early planning stages (only mentioned it to one or two of players in the group), and I've run into a few obstacles to overcome. One is that none of us have ever played a Fate based game, the other is I'm the only one in the group to have read the Dresden Files series (or seen the TV show for that matter).

Not insurmountable problems necessarily. But, what I want to ask about here is for your opinions on some of the feedback I've gotten from one of the players when I mentioned wanting to run the game.

I described the system and setting to him a little bit. Told him that the setting was a bit Buffy the Vampire Slayer, mixed with SuperNatural . . . told him "Modern Day where magic is real and there's all sorts of supernatural creatures: vampires, werewolves, fairy and such". He said he was interested, and then told me what type of character he'd want to play . . .  an extra-terrestrial.

Now, mechanically, I'm sure the Fate system can handle generating whatever type of alien he would want to create. I'm not going to tell him no, because aside from being just a dick move, it could be an interesting character option.

Where I'm running into the problem is that "alien from outer space" doesn't quite fit the flavor of the Dresden Files as I think back on the series. We've got the Modern Fantasy Chocolate and he wants to add his Science Fiction Peanut Butter. I'm not opposed to getting Reese's Pieces out of this, but I was hoping that I could get some idea's and thoughts on how to help ensure that the potential character fits with the game?

Now, all that said, I know that there's a lot that still contingent on other things . . . at this point I only have a partial High Concept, I don't have any information on any other characters, there's been no city creation discussed, and such.

Any suggestions?