Acoustic would make an awful third album, career wise. A bonus disc or album would sound brilliant and I'd love it, but it should never take the place of a Studio album like Teenage Dream or One of the Boys, unless she's sick of being famous and wants to dial it down a notch or two.
I bet it's pretty likely that there will be a Fans Edition of this album, with a bonus disc containing acoustic versions of songs like Firework and Part of Me, along with her favourite tracks from the new album though.