Foils are still great investments for many cards in the Tarkir block (Khans, Fate Reforged, and Dragons of Tarkir), as the prerelease foils from these newer sets have kept the set foils from growing as they normally would. However, Wingmate Roc was not among the cards you could receive as a stamped prerelease foil - meaning that the set foil is the only foil you can get your hands on. You can still find foils of the Roc for as little as $6 shipped, and they average around $7 a pop. There's no way that this tiny spread on non-foils to foils can continue, and there's more money in the foils.
The only issue long-term with Wingmate Roc is that it's not really Eternal playable. It's a bit too much of a mana investment for Modern, so this would be a short-term investment. It does see some fringe play in EDH/Commander, but there's not enough demand there to affect prices all that much. With Khans of Tarkir rotating out of Standard in Spring of 2016, you'll want to only hold onto copies after any price spike if you're playing them.
Risk: Low (foils)/Medium (non-foils)
Price Target: $10 (non-foils), $12+ (foils)
Any cards you think we should be watching? Leave a tip in the comments!