When it comes to Sakurai, I couldn't even tell you. Whether he does or doesn't isn't something I'd like to make assumptions about since assuming what Sakurai would and wouldn't do is normally just a bad substitute for a good argument.
With that being said, the case for Donald and Goofy has always been a flimsy one at best. As you've stated yourself, Mai was kept out despite her importance to the Fatal Fury series. This alone makes a strong case for Sora joining without referencing either Donald or Goofy, since it shows that Sakurai is willing to cut side characters, despite their importance, if they're not appropriate for Smash. And that's not even factoring in the Kingdom Hearts characters that originated from the series (who are all Disney characters, by the by) and the fact that Sora has appeared in 2-3 games that doesn't even feature them.
If I were forced to put a number on the odds that Sakurai would ignore Donald, Goofy, or any of the others, I'd say... About 75% based on the reasons I listed. But again, assuming what Sakurai thinks is not a good argument. I say this from experience after dealing with people who pushed it senselessly against Mii fighters, Palutena, and Robin during the Smash 4 days.