This is what I get for posting stuff without considering it from every angle.
Well, I can't say I didn't consider this, really -- I've just had other thoughts, and as long as I'm mulling things over I figure I may as well think aloud, as it were.
I'm rather set on pairing Elements and aspects, because I like the interaction of the Elements themselves (Wood parts Earth, Earth absorbs Water, Water douses Fire, Fire melts Metal, Metal chops Wood) and the ways that can be reflected mechanically. It occurred to me that the issue is when those Elements/aspects come into play, and how.
For example, let's say you attack someone with your "Thousand Blades" style -- your Kung-Fu aspect, associated with Metal -- and pay a Fate Point to invoke it for a bonus. By default, you get a +2 out of that. Now let's say your opponent defends with his Devastating Wind style and invokes its Fire-based aspect. Does your +2 become a +1 retroactively? Or is it enough that he gets a +3 instead of a +2? What if you didn't invoke your aspect, but he invoked his. Does he get a +3, since your kung-fu's Element is Wood, even though you haven't "officially" brought your kung-fu's aspect into play?
It's a little too wobbly for me.
So what I'm thinking is this: Every action is associated with a Virtue, and, by extension, its Element and aspect. If you decide that you're attacking that Devastating Winds guy because you don't like the way he's treating your father, you're acting out of filial piety (or Xiao), so the appropriate Element is Wood. Now, even though you haven't invoked the aspect attached to that Element -- let's call it "Good Son" -- you're still "using" Wood. Your opponent defends with his style, a Fire aspect. So far, nothing really unusual. But if you pay a Fate Point to invoke "Good Son," you'll only get a +1 out of it. And if he responds by invoking his "Devastating Winds" aspect, he'll get a +3 out of it. In fact, even if you don't invoke, he still gets the +3, because he's using Fire and you're using Wood.
What I really like about this whole channel-every-action-through-an-Element thing is how it puts your motivations front and center. Are you leaping into combat because you're justice at the point of your sword? Or because you're defending an ally? Or because you're fulfilling your duty?
So now then.
Here's the real question. If you're acting out of Xiao/the Wood Element, do you have to invoke your Wood aspect, or can you go with something else -- say, Water, because you decide you're acting out of propriety (i.e., you don't want to lose face by letting the guy belittle your father in public)? Now if your opponent invokes his Metal aspect, does he get a +3 or a +2? I mean, which Element "counts" here? Or are you required to invoke the aspect related to the Element/Virtue you're using? That would simplify things, but is it fair to take away some of the player's choice?
Or does that choice of Element become "your" Element for the conflict? I.e., invoke whatever aspect you like, but for this fight, you're Wood, because you're acting out of filial piety -- so invoking a Metal aspect against you will always net a +3 bonus. After all, this is about your motivation, and that's not likely to change every few seconds.
That seems reasonable, but it also unfairly penalizes the attacker. The defender will always choose an Elemental aspect that's strong against the attacker's Element, whereas the attacker's just stuck with his choice. That actually discourages action, and that's the exact opposite of what we want to do here.
How about this: Take five index cards and write a Virtue/Element pairing on each one. When conflict begins, each participant puts one card face-down on the table, then all are revealed simultaneously. It's an additional fiddly bit, I guess, but it's the only way to do this fairly.