No, I don't think a Zelda spin-off is in the works by Retro for a few reasons!

1st. Nintendo EAD usually handles the Zelda series inhouse directly overseen or with the involvement of Shigeru Miyamoto.

2nd. Retro Studios said multiple times they wanted to work on something new, that leads me to believe a spin-off is unlikely.

3rd. Zelda isn't the kind of franchise to thrive with Spin-offs

Now that all being said I think RetroStudios is being placed on IP creation duty. I don't think Nintendo aquired RetroStudios to simply having them polish old franchises. Nintendo bought them because the studio had three projects already underway on the GameCube. RetroStudios showed an ability to create new quality content and that is why they were aquired (My own conclusion)!

Nintendo doesn't just pick anybody up. Nintendo Of America's job is to target the North American audience and Metroid Prime was that title. MetroidPrime was a shooter/adventure and RetroStudios showed an ability to create an A-class shooter.

So what do I think Retro is actually up too, creating a new shooter IP for Nintendo. Nintendo lacks a solid shooter IP and with the Wiimote so fitted for shooters and Nintendo hinting various times that it was build for shooters. It wouldn't make any sense unless Nintendo had some internal plans for their own shooter.

So Retro I'm 80% sure is making the next Halo, Nintendo's Halo!