At this point, I can't think of a brand new, significant, good addition to the series (emphasis on good; I'm looking at the people suggesting a create-a-character mode, which would add nothing to gameplay and leave us with characters that would inevitably not be as high-quality as "official" ones - not to mention it could lead to the devs half-assing on the roster thinking the CAC would make up for it), so I don't know what Sakurai could bring to the table at this point. For the original, he obviously designed the unique and fresh gameplay. For Melee, he changed and added mounds of mechanics, and he basically made it fulfill the huge potential SSB had (gameplay-wise). Then in Brawl he added the single-player he had been hoping to put in Melee (fortunately Miyamoto convinced him that at the time, focusing on multiplayer was a better idea...smart man), and the online mode Iwata had been planning on basing the third iteration of Brawl around before Sakurai came with his ideas for new things (makes you wonder, just how robust of an online mode would've "Iwata's Smash" have? Pretty robust I would think, considering he was going to make a "Melee 2.0" centered around it...heheh, funny how that worked out), but he didn't really add much to gameplay.
So we have a multiplayer that is pretty much perfect, and a singleplayer that pretty much surpassed all expectations of what a story mode would be like in a fighting game (there's of course lots of improvements that could be done, and lots of people expected a lot more from SSE simply because Smash is such a big, high-quality series and people tend to get a little too excited about it)...all the lovely extras are there, online mode, stage creator, an amazing amount of collectibles, a great soundtrack that encompasses much of, if not most of Nintendo's history...crossovers not only between series, but between companies...
At this point, to me, it seems like the series has gotten to the point where it will mostly be about refinement, and rejuvenating (or putting spins on) the old stuff. There's improving all the multiplayer options and settings and modes. There's improving the online service. There's improving the flexibility of the Stage Builder. And then there's renewing all the old stuff; making a new singleplayer bout, more and new collectibles, remaking old and choosing new characters...and of course the most important thing of all, redesigning gameplay mechanics.
Sakurai would probably call doing this just "adding meat" to the game (probably with the exception of the last detail), but I think many would agree with me when I say that with a concept as good as Smash, that's a good thing. This is just one of the views of his I've heard of lately and disagreed with...there's also his thoughts on competetivity. What things make for a good game in some ways. He's a brilliant designer when it comes to concepts, but I don't think he's necessary for deciding on the "meat" and some of his ideas just aren't that great.
Oh, and I remember when Sakurai mentioned he was hiring a team who had played thousands of hours of Smash...I remember seeing how people thought that this meant they knew everything about the game, and that that could only mean well for the intricacies of Smash. I'm sure it did, but it went to waste because Sakurai's ideas come first.
...I want to say Iwata, because he probably wouldn't mind fattening the game up because that definitely ensures sales. But then again, so does accessibility on some level, so maybe he'd agree with Sakurai's ideas about not alienating players (y'know, except for competetive ones). That just seems to be how Nintendo plays nowadays.
So I guess I'd want the game to go to a 2nd-party developer. Preferrably back to HAL Labs. They created the game to begin with (they even own part of the franchise, right?), they're obviously familiar with it and hopefully they realize that it's much easier and better to make Smash welcoming for both casuals and competetive players than it is to fashion it just for the former (Melee turned out the way it did when they made it despite Sakurai running about with his equally awesome and silly ideas, right? ...although I'm starting to think whatever competetive potential Melee had was a fluke at this point, especially with Sakurai eliminating most of the exact things that allowed for it in Brawl).
