I found this comparison on Twitter. Thought I'd share my thoughts.
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Personally, I think neither work as "the new Ridley" or "the new K. Rool".
Ridley was a hot topic for debate over the years, as many people questioned whether or not a character of his size and build could actually work in a game like Smash. Neither Waluigi or Isaac have that same level of questioning. No one is denying that either could work in Smash, and the only reasonable complication unique to only them that they could face is a lack of moveset potential, something Sakurai's demonstrated time and time again that he can overcome this if he really wants to.
K. Rool is less of a long shot, but it's still not a solid comparison for either Waluigi or Isaac. Donkey Kong (the franchise) is an extremely iconic property, and many felt as if it was long overdue for a third character. Super Mario is also iconic, but no one is complaining about underrepresentation (if anything, people think it's
overrepped). Unless I'm missing another variable here, Waluigi supporters have no other incentive to specifically push for him aside from getting someone they like in Smash. Now Golden Sun is a series that people would love to see get more representation, but it's far from iconic. Unlike K. Rool whose support was far more broad, Isaac supporters are mostly limited to those who are familiar with his series and those who are rooting for another entry. He lacks name recognition, and I feel like that holds him back quite a bit more than K. Rool.