Responding to the OP:
If Nintendo really wanted the Wii U to be a system for the core gamer, then they should have done at least the following three things:
1) Make it more powerful. It should have been close to the XBox 1's power, so that it could run Generation 8 third party games.
2) Just use a classic controller and not include a tablet.
3) Have compelling software for core gamers at launch. Nintendoland was not a title for core gamers, and NSMB U was more for old schoolers. Ironically, the Wii was known as a "casual" console and yet it had Twilight Princess as a launch title. This is the sort of game that appeals more to core gamers.
However, I'm not saying, overall, this would really be a great strategy. The Wii U would have sold better if it had followed the above strategy, but I doubt it would have outsold the PS4. Nintendo is actually better off, overall, following a strategy like the Wii or Switch. In both those cases they had a weaker console, and they fully embraced that fact and that's why those systems worked. With the Wii U, they had a weaker console but somehow wanted to appeal to core gamers anyway. In the end the Wii U didn't really appeal to any gamer, core, casual or otherwise.