Yeah, I agree. Arle is almost [there's some holes we gotta watch out for] an outright lock, especially considering that all DLC, since they come with stages, are much likelier to be from new series and Arle patches up a bunch of holes in the base roster (low female newcomers, low magicians, low puzzle game reps, and SEGA's new rep for this game). Only real competition right now is Joker from Persona 5, as a second Sonic rep wouldn't come with a stage, and most anybody else isn't a big enough deal, or popular enough to warrant inclusion. As for why she'd be picked over him, she fills more holes and no longer is a nobody to the western world [and, there's that whole Nintendo history thing, or whatever]. That being said, I will be the one to say that we're not gonna brigade Joker's support thread.
The fact that so many people forget that SEGA has no newcomers in the base roster, despite being in Smash the longest, really shows how we're a bit of the underdog right now. Almost like Robin in Smash 4, compared to Chrom (Unique Sonic Rep 2), though with us not being bombarded by trolls who would flip out if we got in instead. R.I.P Amitie though, unless DLC fighters can come with Echo Fighters then Amitie is not happening. Also no more Puyo Assist Trophy dreams, but at least that means Arle isn't one, for better or for worse.
Here's a topic: if third parties were in Smash 64, would Arle be one of them? I would say yes. Arle was a freaking household name in the only country that mattered (in regards to Smash) at the time, and at the time SEGA (in 1999, Puyo Puyo was already owned by SEGA, but Compile still got to make games for it) wouldn't have really cared since it wasn't like they were asking for Sonic. Compile would appreciate the revenue boost. However, there is the possibility that Arle would have been cut from the US and European releases for reasons that almost got Marth and Roy cut from Melee's localization. Melee no, Brawl absolutely not (there was no way SEGA wasn't getting a rep that wasn't Sonic) and Smash 4 we know the answer.
Shadow and Akira's sacrifices are what allowed us to get this far. Now, it's time for us to go further than that.