Well, Sakurai has said no Manga characters, even those who appeared in video games. But considering the fact that he changed his mind about Ridley, Pac-Man and Villager and the video game only statement was during the Smash Ballot, he could very well lift the "no other media characters" rule for Ultimate. He wanted to put James Bond in Melee, so he was clearly open to non-video game characters at the time. But the difficulties in acquiring permission from MGM and Pierce Brosnan and the Ian Fleming estate might be why Sakurai would be reluctant to include someone who is owned by more than a game company.
Just to clarify once and for all, did Sakurai ever make an actual rule about Ridley, Pac-Man, and Villager? These are the only references I could find that would be relevant to Ridley's dilemma.
"I think that would probably be impossible.[laughs] If we had put our best efforts into it, we may have been able to do it. But he might have been a little slow. Would that have been all right?"
and
“I definitely know that Ridley’s a much-anticipated name for fans, but if we made Ridley as a fighter, it wouldn’t be Ridley any longer. It’d have to be shrunk down, or its wings reduced in size, or be unable to fly around freely. ...Providing accurate portrayals of characters is something I want to pay ample attention to. ...If I don’t stick to that thought, then we’d have to lower the quality or break the balance of the game. Something that goes way off spec could break the entire game.”
This seems to be less about a philosophical rule, and more about a technical limitation. Sakurai expressed confidence he could make Ridley work if he and his team buckled down and made a concerted effort, but that he didn't know how to make it work at the time. This is different from a philosophical statement about what he believes Smash Bros. is supposed to be, such as the remarks he made in reference to manga characters:
“Manga characters will not join the battle (obviously). While it’s true that we’ve added characters from other companies,
It’s not going to be anyone and everyone."
Ridley's problem was a technical problem, not a philosophical problem. So to say that Sakurai might add Goku because he "changed his mind" about characters like Ridley or Villager seems to me to be a false equivocation. Sakurai's never given any indication that he has any intention of changing Smash Bros.' identity. On the contrary, these characters only came about because they found a way to conform to Smash Bros.' identity. In his statement on Ridley:
"I definitely know that Ridley’s a much-anticipated name for fans, but if we made Ridley as a fighter, it wouldn’t be Ridley any longer" and
"Providing accurate portrayals of characters is something I want to pay ample attention to..." - we have strong evidence of just how much Sakurai cares about the philosophical integrity of the game he has created, even if it is at the expense of what the fans want. When Ridley finally happened, the character was made to work in such a way that satisfies the essence of the character, and feels natural for Smash (an incredible accomplishment, honestly).
Smash Bros. is about Nintendo. Every character that has been introduced so far has had a part in shaping (or being shaped by) Nintendo's rich legacy, in one form or another. I do not see how Goku, or any other fourth party character, contributes to preserving this focus. Introduce Goku as a viable contender, and people's attention will shift from the likes of Lip, Balloon Fighter, Takamaru, Mach Rider, and other untapped goldmines of Nintendo's rich legacy, and become saturated by empty requests for characters like Shrek or Batman - which sap away Nintendo's spotlight and have little to do with Nintendo itself, and turn Smash Bros. into something entirely different. It is good that the focus on Nintendo is largely still preserved on these boards - it made the likes of Pit, Ice Climbers, Duck Hunt - and Fire Emblem itself - among others, possible (even probable) in the first place.
To clarify, I definitely think there is a place in gaming to see a showdown between Mario and Goku happen, and I don't have anything against Goku fans (secretly, I think it would be a lot of fun to see Goku in Smash). But I don't believe Super Smash Bros. is the game to make that happen. And I believe the game itself would stand to lose a lot of its soul if it were to happen.