I have to confess I don't have a good idea what director does do and what he doesn't (directly) do in case of game as huge as Smash currently is. So let's assume that Sakurai does the following:
- Roster selection
- Character movesets
- Global mechanic selection
- Stage selection
- Modes selection
If you see some kind of huge error there, then go tell me. (I guess that he creates some items etc., but not all of them, so they don't count.) Here's my personal thoughts on this stuff.
I'm completely satisfied with Sakurai's rosters so far. Not that I wouldn't swap some characters in and out, but I wouldn't expect anybody to perfectly satisfy everyone. Moreover, I think many people wouldn't include unexpected contestants (like WFT, not necessarily WFT per se). However I think it wouldn't be hard to find someone else that would make a great roster. I'm a little less satisfied with stages, but given that your usual franchise has loads of locations available, it's not that surprising that it's very hard to manage them all; again, I think it wouldn't be hard to find someone else selecting stages well.
In case of game modes, Brawl wasn't exactly perfect, being overfocused on story mode and specifically not so wonderful story of story mode. However, given other games with Sakurai's influence, I think this was an incident that won't happen again. I suspect he simply wanted to make a game with good story playing a major role and he failed with the "good story" part, but then he tried a different type of story in KI:U... and I guess he succeeded? Comparing the two gets surprising. Both follow rather serious line of major events, when you think about it, and both try to keep it lighthearted somehow. KI:U uses lighthearted dialogs, SSBB uses no text and thus has to resort to lighthearted minor events. Maybe this has some deeper effects. I don't know.
Character movesets are the point which I would like to change quite a lot. Sakurai as of SSBB liked to ignore some part of a character and at the same time to make up some ridicoulus moves. We'll see with SSB4 will it change. If it doesn't, a change would be very welcome.
What's left is global mechanic selection. This is the point that has greates potential to get @#$%^& up by a new director. And this is a very important point unfortunately. SSBB did not include major new mechanics except for Final Smashes, it even cut on some. However SSB should not transform into some hardcore-only fighting game in which the player has 1300 possible moves and 20 gauges to evaluate at any moment. Lack of this is part of its uniqueness. This does not mean all new mechanics are bad, mind you. But it's very easy to overblow that, and Sakurai did keep this aspect of the game well.
I guess I'm indifferent overall.