Oh please, don't get overboard again... Mario Kart 8 having oddball characters was the exact reason I skipped the DLC on Wii U. Excitebike Arena was an awesome course that I felt fit due to it being an old Nintendo racer (a track from F1-Race or Famicom Hot Rally would work for similar reasons) but all the Splatoon, AC and Zelda characters were just silly additions, especially considering how the only Mario characters we got were reskins of other characters. The amiibo costumes in that game served their purpose, so making it an abrupt crossover was dumb by adding more outsider characters over Mario ones like E.Gadd or Fludd was just silly IMO.
I hope this is a one off thing for MTA, since not only does it have the issue of on cart DLC to deal with (though this certainly wasn't one of it) but it would lead to yet another Mario game being filled with more non-Mario characters rather than more interesting Mario characters.
What tennis series does Nintendo even have besides the Mario one? At least MK8 had a lot of racing crossovers like Big Blue and Excitebike, but if they wanted to bring a character that's from another Tennis game Nintendo made, the only one I could think of would be the old NES Tennis player... I guess Link would be a crossover character in that regard since he's from Mario's sister series, but still. Very oddball choice indeed.