I may be wrong - as I'm almost every second - but I guess that sadly (or happily), the NX can, in fact, be a substitute for both, Wii U and 3DS.
I mean, it will be like a PS4 to the PS3, or a Xbox One to the Xbox 360. Nintendo won't abandon Wii U or 3DS, they will continue receiving new games (at least for a while), but I think that after the NX, we won't see a "Wii V" or a "4DS".
Said that, I think that is pretty fair for us to think that the eight generation of the main Pokémon series could be released for the NX (and also the remakes of Diamond and Pearl).
Talking about the games we're going to see on the NX, as May told, the new Sonic is one of titles that I'm looking forward to buy with the console (once I have enough money - and will - to buy the NX). But, the thing I'm really begging Nintendo to do, make a console interesting for the third parties and bring back great "real car" racing games, something that I miss since World Driver Championship (aka "The Gran Turismo for Nintendo 64).