Yo, fellow fighter here!
These are not Wii U titles per se, but the Wii had a fair share of goodies that'll keep you entertained on your Wii U as well:
1. Guilty Gear XX Accent Core (Plus)*
2. Tatsunoko vs. Capcom (Wii exclusive)
3. King of Fighters Collection: The Orochi Saga
4. Samurai Shodown Anthology
5. Bleach: Shattered Blade
6. Naruto Clash of Ninja series
7. Mortal Kombat: Armageddon
8. Cartoon Network: Punch Time Explosion
I may have missed a few, but to say the Wii has no fighting games would be stupid. Wii U adds a few to the list such as Tekken Tag Tournament 2 and Injustice. More may come, Arc System Works even showed interest in a Wii U port of Xrd if there's enough demand. If you're a real Guilty Gear fan you'd not wait for Xrd, though. Did you not see those dust attacks in the announcement trailer? Or the priority guard system? Yuck!
If you want a great system for fighting games then your best bet is either a PS3 or PC. (If only for more options in buying a good arcade stick.) Some of the better stuff is available on PS3 like BlazBlue: Chronophantasma and Persona 4: Arena. Xrd is also maling its way to PS3 if you're really, REALLY, into it. I'm personally not and have played Guilty Gear since the first version on PlayStation. Far prefer #R to any of the current versions, but Accent Core Plus is very good.
*This one supports 480p and since it's 4:3, upscaled on a Wii U via HDMI it looks gorgeous. It's as sharp as Accent Core Plus (PSN version) on a PS3 via HDMI. I play on a 46" Sony BRAVIA TV and it's definitely Guilty Gear on my Wii U over the PS3 any day. Just remember to play with a Classic Controller Pro (which to me feels more precise than a DualShock 3) or if you want an arcade stick look on eBay for the Tatsunoko vs. Capcom stick and mod it with Sanwa parts. It's made by Mad Catz and is super cheap. It also has a switch that changes the behaviour of the joystick (to have it simulate the D-pad or the analog stick on a Classic Controller, making it compatible with Smash).
Hope this helps.