On ABXD Mini, I have an email field, a public email checkbox, and a gravatar checkbox. It has no avatar linking or uploading, only gravatars. If you provide an email address, no matter the public/private switch, and enable gravatar, it'll try to use that email address. Hashed, of course, as Gravatar wants. If your email isn't public, your profile page won't show it.

So "kawaoneechan@gmail.com" → "d0bd59922f5096b2c559f2b3a5d6ded9" →

If I wanted to have both gravatars and moods, I'd probably take the current system (upload your primary ava and any moods) and add a "use gravatar" checkbox. If you pick a mood, it'll use the one you uploaded, if you pick "-Normal Avatar-" it'll use... either the uploaded pic, your gravatar, or none if you so happen to have no normal avatar set. That's what I'd do.

If gravatars are enabled && email is set, use gravatar src, else if picture is set, use picture src, else print nothing.

bam you're done. If it's overcomplication, it's not a whole lot.