It being Inklings is more so a theory based on what several of these Twitter guys have been saying about the alleged newcomer (first party character, not a ballot choice, makes sense, will likely cause a lot of social media buzz etc.) couple with Splatoon looking likely to receive a major update soon which would coincide nicely. No one has actually come out and said who it is though.
That said it could arguably apply to some other characters. K. Rool checks off some of those boxes I think but I still think it's more likely he'll be a ballot choice since I don't think there'd be much basis of Nintendo jumping onto his development for the October-estimate juncture without having seen the full fan turnout, so I look forward to hopefully seeing that pan out.
I think it could be K. Rool if they saw him zooming ahead early on, and saw how popular and highly requested he was even before the ballot as well. Simply seeing all the requests for him online, they could've been "we just need a little bit more info, how will be fare in a worldwide ballot?" and if he also did well there within the first month, he's a for-sure set pick.
Inklings, even if started at the earliest possible point, would likely not be ready in time for October. Why?
Clearly they had no intention of doing Inklings at the time Splatoon launched, otherwise they wouldn't have made the costumes, they would've said something like "stay tuned for Inklings coming in Smash!" If anything, they would've planned Inklings for Splatoon's launch, and Nintendo did not realize it would be such a massive success, either. By E3 they knew it was popular, but that still likely wouldn't be enough to justify it yet. They'd have to wait longer and see it skyrocket before finally deciding, and by roughly the end of June/early July is likely when they'd finally settle on creating Inklings. And that's simply not enough time for a mid-to-late October release. That'd have to be November or December.
Maybe they did them earlier, but the timing just doesn't work very well. Whereas K. Rool, they would've known he was popular, but maybe didn't anticipate precisely how popular he'd be, but if he skyrocketed to the ballot right away, that's when they decided him, an easy month or two (early-to-late May) in advance of them deciding to do Inklings. (They probably finished Ryu, Roy, and Lucas by May, so that's when they'd need to start another new character as well).
If they announced a character 1-2 months in advance I could see it being Inklings, but this seems to be implying the character is likely to release very soon, probably by the end of October, and that's a bit too tight for a brand new character I think.
But that's just me. And yes, I do believe they started Ryu once the game launched. Roy and Lucas might've split the development time during this point.
