Honestly, out of Isaac and Starfy, it would depend on if Sakurai's looking more at pre-Brawl popularity or post-Brawl popularity. If pre-Brawl, then Isaac by far. If post-Brawl, I feel Starfy has the upper hand.
I wouldn't be so sure. If Sakurai looks at post-Brawl Japanese popularity, then yes, Starfy definitely has the upper hand, but pretty much every other situation Isaac has more popularity. In the west post-Brawl Isaac has still remained a pretty popular pick, he's still in the top 10 (well, he was like last month, I dunno if he still is Embarrass) and Starfy has fluctuated a bit, and doesn't have low popularity or anything, but still outside Japan, whether pre or post Brawl Isaac has always had more.
Well they've both been without an installment for about the same amount of time, its just that Starfy has the advantage with Japan popularity, but Isaac, overall, is more well-known. When did Isaac's popularity plummet and Starfy's skyrocketed?
Isaac's popularity in Japan plummeted pretty soon after Brawl, but in the west his popularity has remained relatively intact, though ofc it's not what it once was. And Starfy's popularity has never "skyrocketed" per say, but around when his last game came out was probably the peak of his popularity. Ofc that was just after Brawl, waaay before Sakurai started compiling this roster.
Explain why. He's certainly more likely than Golden Sun or XenoBlade.
How so? Just because he has more games doesn't make him the unquestionably more likely character. GS overall has still left a bigger impact and Shulk is a much more popular pick in Japan (where Starfy has the highest popularity). Not to say he's "unlikely", just that he's definitely not leagues ahead of Isaac or Shulk.