There's discussion of a lot of previous series that already are in the game. My opinion: most of them have the characters that matter. Would like Zelda to have new characters, perhaps, maybe trident pig Ganon instead of FalconDorf, but I am aware people are averse to Smash characters having big changes.
That all said, I'm personally more interested in new series, first or third party, though third parties are very much more likely to drive up interest at this point. Still, thinking on it, the major recurring Nintendo series that don't have characters in yet, off the top of my head, are ExciteBike, Nintendo/Advance Wars, Panel de Pon (to an extent), Custom Robo, Nintendogs, Rhythm Heaven, Style Savvy, and if you really stretch it, Golden Sun, Starfy, and Chibi-Robo. Some of the characters to add from these feel obvious if you know of them, but Nintendogs always feels weird to think about. Either you have realistic dogs, or the trainers most don't really think about, both of which seem odd as inclusions for different reasons. A stage may be adequate in most people's view, but perhaps a guy throwing a frisbee and dog bones at people could be kind of funny. I think if I had to choose just a few of the series I mentioned, ExciteBiker, Andy or Sami (Advance Wars), Lip (Panel de Pon), Ray (Custom Robo), and any sort of Rhythm Heaven character have the most clear and/or unique options as far as I can envision. This is all to say, I don't think the Nintendo well is dry, but it is more in obscure places in my mind rather than sticking just with series that already have a decent amount.