See, I don't get this excuse. Especially when you say the last console you owned was the N64.
Firstly, starting with just games from this generation: did you play through Captain Toad?
Donkey Kong Country: Tropical Freeze (expounding on that, did you play Donkey Kong Country Returns? I assume you already beat the SNES DKCs, now on Virtual Console)?
Mario Kart 8 got life injected (like it needed it; it's Mario Kart, after all) in it twice with two incredible DLC packs--you bought those, right?--but even the vanilla game is fun on its own. I've yet to get three stars on everything.
Mario Party 10 is really fun with a group, or even just one other person.
New Super Mario Bros. U and it's DLC? I've yet to get started on that, because I haven't played through all of NSMBWii yet (another you missed).
NES Remix 1 and 2? Lots of challenges there.
Nintendoland is an incredible party game, but has a bunch of solo challenges, too.
I assume you got Rayman Legends for another console, but if not it's worth playing on WiiU!
Super Mario 3D World is incredible, and challenging as all get out. You've 100%'d that for sure?
Super Smash Bros. for WiiU has so much content I can't believe you've already finished it.
Since you missed out on GameCube era, Wind Waker HD is a must.
LEGO City Undercover is a very fun, GTA-esque (but, LEGO) exclusive.
Then there's the ones I either haven't gotten to, or have only spent a brief amount of time with: Game & Wario, Pikmin 3 (you'll need to play Pikmin 1 & 2 for GameCube or Wii!), ZombiU, The Wonderful 101, Hyrule Warriors, Bayonetta 2, Kirby and the Rainbow Curse, Pushmo World, Affordable Space Adventures (last two on eShop) and more.
Then there's the Wii titles you missed, like the aforementioned DKCR, Epic Mickey, Metroid Prime Trilogy (available on eShop!), the aforementioned NSMBWii, Super Mario Galaxy, Super Mario Galaxy 2 (available on eShop!), Smash Bros. Brawl, LoZ: Twilight Princess and Skyward Sword, Wario Ware: Smooth Moves, Xenoblade Chronicles will take months to complete. Some that I haven't gotten to: Donkey Kong Jungle Beat for Wii, Kirby's Epic Yarn and Return to Dreamland, Fire Emblem: Radiant Dawn, Wario Land Shake It, Sin and Punishment: Star Successor, Pandora's Tower, The Last Story, Punch-Out!!, Rhythm Heaven Fever...
And then there's the entire Virtual Console catalog that you probably already played through. Did you own both GBA and DS? If not, those libraries are expanding on VC.
Then coming out this year we have StarFox, Yoshi's Wooly World, Mario Maker, Splatoon, Xenoblade Chronicles X, whatever the hell else they announce at E3. Maybe GameCube titles on eShop, which I would go down that rabbit hole but I'm probably about to hit my character limit.
People act like there's a dearth of content, but either you have way more free time than I do (and I have a reasonable amount of free time), or you're just not looking at what's available to you. I don't mean to sound antagonizing or anything, I just literally don't know how someone can be caught up. There's only so many hours in a day. And I've got a 360 and 3DS backlog, as well.