Why I write what I write? Or what make me pick which fandom to write for, out of all the ones I like? (Stolen from the discussion back in January on ffr_discussion (What gets you writing?))
The following points are not applicable for all fics, but they represent majority of the fics I've written or will write.
1. Has someone I can (and want to, of course) gender-bend (usually male to female, but female to male is also possible), even if I never end up writing anything on that topic at all (not very likely now, but I've restrained before). (My definition of gender-bend is actually pretty narrow. Cross-dressing (without a real physical transformation) does not count. AU where the character is always the opposite gender does not count. Getting permanently stuck does not count (but it's fine if the character him/herself believes that they might be permanently stuck).) That someone is usually the main character (see #2) but can be someone else, too (usually either the rival or the sidekick). So far I've gender-bended main characters (well, one of them is more like main character's love interest) from four series (all male to female) I have talked about before on this journal, not really counting any series where gender-bending or body-switching (preferably when the switchers are of opposite genders) or spiritual possession (same deal as body switchers) is already canon (even if any of those things only happened once in the entire series).
2. I love the main character. That might be obvious, but for me it is very important. The main character represents/stands in for the major portion of the series to me (i.e. big reason I tune in at all), so if I don't love the main character, I will have a hard time getting inspired, even if I like other characters and the main character might not feature in the fic I want to write at all. (Not anything new--if you read my journal for a while you already know I'm a main character person.)
3. Usually (but not always) has more than one incarnation, so I can pick and choose parts that work for me (i.e. parts I like ^^;;) from different versions of the same series. I mean, for example, a series can have a manga version, and an anime version, and I might prefer some parts of the anime better than the same parts (or not the same, since maybe manga doesn't have them) in the manga and vice versa. (Or a manga version, and a live action version (drama, or tokusatsu, and/or movie). Or maybe a light novel, a manga, and an anime. Or maybe some other combination.) But I'll usually say I base a fic on one of the versions only while hoping the one I choose is predominant.
4. Has to be something I can make up stuff with ease and can fool myself into thinking that it fits the original series (or not too out of place). So historically based series are out for me (not that I really follow any except Gintama), because I don't much feel up to pretend to know something I actually don't know, and in-depth research is a bit too much work for something I do in my fun time.
5. Has characters I can understand, or at least can pretend to understand. Kind of a subset of the previous point (and an extension of #2--about the main character). So, in other words, the series I write for better has characters I like, because if I don't like or feel too indifferent for most of the characters, I might not care enough to even try to understand them. And I do not write about characters I cannot understand, since it would be very unfair to them, and to the fans who actually understand those characters. That is not to say that I will never write about characters I might not be too fond of--the main point here is the characters simply have to make me feel invested enough to at least try to understand them.
(I probably could have said #5 in far less words, but don't feel like editing myself right now...)