I think the future of 3rd parties will depend largely on where the economy is 2-3 years from now. A large part of the reason why 3rd parties went multiplatform this generation is that the economy tanked, so these large-budget games need to sell on more than one console to turn a profit and Sony & Microsoft were less-willing to pay for exclusivity (the new battleground is DLC-priority and download title-exclusivity). If the economy stays as it is (or gets worse), I would expect 3rd parties to stay as multiplatform as possible. If the economy picks up, 3rd parties might see more of a benefit to platform exclusivity. I think 1st party is still going to be the determining factor for platform purchases.
As for the Wii U's tablet controller, I wouldn't expect more than a token effort by 3rd parties to take advantage of it. It's just not worth the cost to devote a large number of resources and manpower to really taking advantage of the thing if your game is going to sell as poorly as 3rd party games normally sell on Nintendo consoles. I would only expect that to change if Wii U software sales dramatically exceed historical expectations. Even the Kinect, as well as it has sold, has barely had more than a token effort of support by 3rd parties so far.