as you may have noticed, the most important thing is to figure what your real feeling about that girl are. i mean if you just like her as a friend or if you want something more.
from what you have written, i think that you feel something more but you are just too afraid to go for it. well, DONT BE! all these questions, dark visions of ruining the friendship, all those "if" questions, they are all making a mess in your head and you end up doing nothing. If i were you i would try to find out what that girl thinks about me. If she has similar feelings to me, then it may work out, right?
So, first of all - find out what she thinks of you, does she consider you as a friend only or a candidate for something more? and then, if it turns out that she may have crush on you, go for it. Just ask her out. You have nothing to lose man, and you have a lot to gain.
PS: you say that you dont wanna ask her out yet. well, why is that? you just got to make up your mind if its because you are afraid of destroying the friendship or if its because you dont want her as your girlfriend.
Dont let the fear make you quit, because after some time you may just regret it! Do something before its too late!