I can't see a Wii U exclusive title happening, but I can see a multi-platform Sonic title with Wii U exclusive features (a la All-Stars Racing Transformed). Wii had titles like Colors and the storybook games because it simply wasn't possible to port games like '06, Unleashed and Generations. The History of Sonic book (highly recommended) made it pretty clear that Sonic Team's intent was to have Sonic's games be multi-platform with Wii/360/PS3, but '06 taught them that this wasn't possible which lead to Secret Rings. They tried to do what they planned on doing with '06 with the WiiS2 Unleashed, but I think the PS2/Wii version was just effort that would have been better spent on a Wii exclusive title. I think gamers saw the SD version of Unleashed as a gimped version and so did Sonic Team staff. Thus explaining why they just up and made a brand new title with Colors when Generations was releasing the following year to the HD consoles. I don't think Generations was ever intended to be made for Wii, as Sonic Team already learned their lesson with '06 and Unleashed.
The only time I think we'll see Nintendo exclusive Sonic games is on the 3DS, and Mario & Sonic on the Wii U.
Now they don't have that issue, since Wii U is at the 360/PS3 level. So my prediction is:
• Boxed main series game for 360/PS3/Wii U - Wii U gets features that 360/PS3 doesn't have, and it ends up being the last main series Sonic game for the 360 and PS3. Sonic Team will develop the following main series title for the 720, PS4 and Wii U. 3DS version may or may not happen, given ASRT and possibly Mario & Sonic releasing to 3DS in 2013. Personally I'd like a 3DS platformer that isn't a handheld version of a console title. Rush and Rush Adventure were great, Colors and Generations 3DS were good but don't beat the Advance or Rush series.
• Mario & Sonic 2014 Winter Olympics for Wii U/3DS - not sure if SEGA's olympic contract ran up or not. I could see this either releasing late 2013 (in line with past Olympic titles) or early 2014 (more dev time due to it being a new platform)
• Sonic 4 Episode 3 - this is 50/50, but I'm leaning towards it happening. SEGA is focusing more on digital, and I think Episode 2 made a profit.
• Sonic rerelease(s) - likely candidates are Chaotix, Sonic R and Sonic Heroes.
I'm *hoping* though that it's in a larger, more "action/adventure" game-style context similar to Unleashed's hub worlds.
I've been playing Sonic Rush Adventure for the first time this past week, and I'd love to see the island system implemented into a game. The island concept of the Sonic series has always been cool to me (South Island, Westside Island, Angel Island) and I'd think it would be cool to have a sandbox style map where you can fly about in the Tornado to reach various islands (each acting as an action stage). They could either have a free flying control, like Just Cause 2 or the GTA games, or a point A->B style mini game like the gummi ship in Kingdom Hearts 2 or the boating stages in Rush Adventure. The key would be to not make these annoying (Unleashed flying stages), too difficult or too long (Unleashed flying stages). I thought the Rush Adventure boating segments were a good length, a few minutes. Perhaps populate the sky with cool power ups and vintage aerial badniks. And of course, once you visit an island, you can jump right to it via a map screen.