Ratings

by understanding the ratings you can decide what your fics will be like

there are many fanfic style sites you can post on fanfiction.net is the main one.

K ratings a kiddie ratings fluff no action ect
T ratings are more action packed may have language fight scenes adventure ect
M ratings are more mature may contain lemons usually censored for this site can contain things of all of the above

other sites like mediaminer or adultfanfiction.net contain M rated and even X rated material

there are also fanfic blogs where you can post any kind of fics Bumwolf.com has opened up a fanfiction section as well for uncensored and X rated works

What Ifs 101

Ok when you ask what if about a series don't get hung up on the little things, like hair, eye color.
Again we will look at Naruto

What if Naruto had black hair? Not very ground breaking and doesn't open much plot

Focus on abilities, places, skills, ect

Like What if the rookie 9 were the vessels of the 9 bijuu.

there it opens up your creativity, ideas, plots and by asking that more questions can be asked and soon you'll have yourself a fic