I think a feature of the NX will be its ability to play Wii U games but also offer updates for those Wii U titles only accessible for those playing on the NX.
For example, I don't think the next Smash Bros game will be built from the ground up, instead I think it will essentially be a large expansion of new content for Smash 4, but the new content may only be accessible if you play the game on the NX. This way, Nintendo could sell the new Smash title at retail as an NX title with everything that Smash game will have to offer, all the Wii U content plus the new large update as a single release for those that didn't buy the Wii U version, and then Smash Wii U owners could just install the expansion on the NX at a cheaper cost than a full new title if it detects you own the game. Seems really feasible to me and I'm kind of surprised something like that hasn't already happened.