My expectations for third party publishers are that they're going to be focused on making "multi-generational" games until the end of 2014 at the earliest, and after that the platforms they will target will be based on the userbases and sales of next generation hardware ...
If the Wii U sells well over its first 12 months you will probably see it become the lead platform for most next generation content released in 2015 and (probably) 2016. I expect this because publishers would anticipate that the Wii U would have the largest userbase of platforms these games were released on when they were released. Beyond 2016 it really comes down to how well the "XBox 720" and PS4 sell.
Hypothetically speaking, if the Wii U averages 15 million units per year over its first 3 years, the "XBox 720" launches in 2013 and averages 10 million units per year over its first 2 years, and the PS4 launches in 2014 and averages 10 million units in its first year it is likely that the bulk of third party games would be released for the Wii U for the rest of the generation; in contrast, if the Wii U averages 10 million units per year over its first 3 years, the "XBox 720" and PS4 both launch in 2013 and average 15 million units a year for their first 2 years, it is possible that the Wii U would soon be dropped as a supported platform for a large portion of games.
Of course, there is also the slim chance that the Wii U will sell at a similar rate as the Wii did for its first 3 years and the "XBox 720" and PS4 will sell at similar rates to the XBox 360 and PS3, and then the question becomes whether third party publishers will heavily support those consoles. After all, at the end of 2015 if the Wii U has 60 million units sold, the XBox 720 has 16 million units sold and the PS4 has 8 million units sold which platform would you target the bulk of your resources to if you were a third party publisher?
