There are quite a few I'd like.
I think the best thing to do is add a rep from a dev/pub that's not yet represented in Smash. For fighting games, the most logical one would be Arc System Works with Guilty Gear. They released it on the Switch and a new one is coming next year (though don't know how likely it is to release on Switch). I'm not familiar with the series, so I can't tell which character would be the most interesting.
They could also do Koei Tecmo with Dead or Alive, but I'd rather have Hayabusa Ryu from Ninja Gaiden as a KT rep (no, he does not count as a fighting game character!). But the great thing about Dead or Alive is there is a good chance for a female rep, with Kasumi being the most likely one. But I wonder how she'd stand out besides her being female.
We could also get a Mortal Kombat rep, perhaps Sub Zero. It's a big fighting game series with history on Nintendo platforms. And it has characters that could be really interesting, like Sub Zero as I said.
If we'd look at reps from devs/pubs already represented in Smash, the most obvious one would be from Bandai Namco. Tekken is one of the biggest fighting game series and the best selling behind Smash. Heihachi would probably be their pick, but I'd prefer Jin (also likely) or Asuka (unlikely, but I'm a fan!).
I like Soul Calibur more than Tekken, so I'd choose SC over Tekken. Mitsurugi, Nightmare/Siegfried or Sophitia might be their pick, but I'm a fan of Seong Mi-Na and Yun Seong. I've only played SC 2 and 3, so I don't know much about any of the newer characters.
In the end I think a Guilty Gear character would be the most interesting one, despite me not being familiar with the series. But I'm not really waiting for any character from a fighting game anymore (I wouldn't mind a Tekken or SC character though). For the last character, I think Hayabusa Ryu would be the most logical one as being from a big game series and publisher not yet in Smash (Ninja Gaiden since the NES and from Koei Tecmo).