The Wii U had a lot of great exclusives that sold very well considering the system's relatively small install base. To this day it has arguably the best library out of any console from its generation. Poor marketing, branding, and an unappealing design were the reasons for its failure, not the library.
Most of the Switch's potential install base has never had a chance to play any of those games. This puts Nintendo in a unique position in the history of the game industry, where they can simply port over their entire Wii U library and have an amazing launch lineup that all of the people that skipped the Wii U would buy in droves, now that there is a non-doomed platform to play them on.
However, it is also important to please and pique the interest of the hardcore fans that bought a Wii U. These people, the ones most likely to become early adopters, will not be interested in straight ports. Launch with nothing but straight ports and Breath of the Wild (which they can always buy on Wii U if push comes to shove), and they will hold off until the true sequels and exclusives arrive. This would deny Nintendo crucial momentum.
One extreme is to port games more or less directly to get them on the market ASAP for the newfags. The other is to develop full sequels to please the oldfags. It is a balancing act to find the right compromise between the two.
For most singleplayer games, I think it would be too much work to add enough content to get all but the most hardcore of autists to buy them twice. Add a few bonus levels, increase the rendering resolution and use the higher quality source textures that I hope they have lying around (assuming the Switch is significantly more powerful than the Wii U, which we don't know for sure yet) and take any repeat purchases you can get.
Many of the Wii U's best selling games are multiplayer, however. Mario Kart 8, Super Smash Bros, Splatoon, and so on. These games people will be much more willing to buy any kind of sequel to, so that they can keep up with the rest of the player base. For these games, I think it makes sense to go the extra mile to make the oldfags happy. For instance, we have already seen a glimpse of a new Splatoon in the trailer with new maps, weapons, and categories of gear (hairstyles and pants). Add enough content like that and promise to trickle out more as DLC like they did with the original game, and they will easily get the fans on board.
They could drive a lot of repeat sales to both categories if they offered reduced price "upgrades" to owners of the Wii U versions of these games. But for multiplayer, they will probably prefer to do whatever it takes to get people to buy the games at full price twice.
Get this blitzkrieg of ports and half-sequels out in time for launch and the following months, and for the love of god include a better game than Nintendoland, and I think they will be successful. Pack-in Breath of the Wild and it will sell like hotcakes.