Still, I'm hesitant to say she's top 5 when I can already think of the top 5 after release: Rex, Springman, Waluigi, Shovel Knight, and Rayman.
If we're referring to DLC then maybe but if we're talking about Smash 6 then here are my thoughts:
1. Rex is likely DLC so he'd be out of the running.
2. Springman is unproven and I doubt ARMS will blow up the same way Splatoon did. Though I could be wrong.
3. Waluigi is a competitor.
4. Shovel Knight hasn't proven that he'll stand the test of time yet. He's still riding high off of his first game (even now) and his several cross-over appearances but only time will tell where the character will go and how loyal the fanbase will remain.
5. Rayman could be a competitor. Though I don't think he's as high up on people's wish list. And though it's seemingly not a factor, being a Western character could hurt him. The roster is still overwhelmingly Japanese (with the Western characters being 1st parties).
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I think the competition is pretty weak outside of Waluigi. There's nothing suggesting her popularity won't grow as well, especially if WarioWare Switch comes out and is commercially successful. At minimum, I expect Sakurai to know about her current popularity and for this to reach Nintendo execs' ears and I expect her to continue her breakout and receive merchandising. Nintendo would be leaving money on the table by not capitalizing on her popularity.
He's already alerted them as to King K. Rool's popularity, which they were (ridiculously) unaware of.