In regards to the latter, it depends just how much kit from Shantae's games she comes with. The debate about her transformations being "Too complex!" is getting silly in light of the elaborate DLC fighters we've been getting so far. On the other hand, Pirate's Curse did help her series set foot in Japan, so they would still need to balance out her classic moves with those from a fan favorite entry (not to mention taking anything at all from Seven Sirens), a challenge that I'm sure they'd feel like taking on.
Also, Ken straddled the line the most in this batch, seeing as he's got a few considerable differences compared to Ryu.
In the meantime, as far as second Microsoft characters go, it all comes down to either Steve or Rash. Master Chief isn't nearly as guaranteed unless the classic Halo games get ported for the Switch, which might happen sometime in the next few years.