Thing is I don't even think that SEGA is at all inept when it comes to Puyo storytelling, I blame what's currently going on entirely on the current formula. I said it before and I'll say it a million times over, but of course the series' narrative is going to become stagnant, when it's always the same five characters every time + whoever happens to be new, you aren't going to get much more out of the returning cast then Ecolo being a fuck-up and Satan's infernal boner for Arle.
Dear God...and this has been going on for, what? 4 games now? Well, I mean...should I count 20th Anniversary?
The rest of the cast has been rendered irrelevant as a result, regardless how much there's left hanging, there ain't exactly much that Sig or Raffine can accomplish in the minimal amount of screentime they are given and others like Rulue, Witch or Draco have done so little since their respective returns to the series that you could easily cut them again and not lose much of anything of note.
It doesn't help that the limited amount of screentime can lead to bad cases of flanderization or some cases make it seem like such a case, because I actually don't think that Rulue for instance has really been flanderized, she was always obsessive and psychotic about her crush on Satan, she just gets nothing to do anymore, which is why that's pretty much all she has now, as opposed to a change in her characterization.
You know, now that you mention it...can we really say that Rulue, Schezo, Suketoudara, or Witch have been flanderized? I mean, you already have a point that due to the limited screentime SEGA gives the cast outside of the usual 5, characters like Rulue just don't really have anything important to do anymore but psychotically lust for Satan.
Heck, Schezo has always pined for Arle's power and was always regarded as a creeper, it's just that (to me) that gag has REALLY run its course over the last couple of years (specifically after 15th or
maybe Puyo 7, if I'm being generous about it).
Witch never really did much in the first place aside from create potions, occasionally screw over customers and anyone unlucky enough to incur her wrath, and attempt to cheat to defeat Arle.
Suketoudara is in the same situation as Witch, not really being important in the first place, but being around for comic relief. Good comic relief, IMO.
Sadly, Draco HAS a severe case of Flanderization to deal with, losing the majority (if not, ALL) of her tomboys traits, being more of a perpetually hungry dimwit. I mean, I still love her, but SEGA...what the heck?
That's the thing, there's only so much you can do with the Arle/Amitie/Ringo/Satan/Ecolo combo and even that has been exhausted a long time ago, the other characters still have plenty things to offer, development to experience and plot points to wrap up, but as long as the series continues to treat them like second rate material we ain't gonna see any of it.
It's like they have all of this material to work from all these dimensions worth of characters and rich lore, yet SEGA won't touch it with a 10-foot pole...
BTW...I don't if you saw this, but I came across something VERY interesting. It's basically about the Puyo/Madou timeline. One thing's for certain...Doppelganger Arle x Sig is no longer a far-fetched ship to me anymore after reading about Doppel's TRUE history and her REAL motives behind wanting to kill Arle. Heck, her very first alt. in Quest can be tied to the lore found on this site! Specifically, the part about how Doppel knows something about Arle that no one does.
That being said, if you could reblog this:
https://puyohero.tumblr.com/post/172317966245/shin-madou-timeline-puyo-puyo-universe-legends
And read this:
http://puyopuyouniverse.proboards.com/thread/43/shin-madou-timeline
No, seriously. Read the "Old Madou world" stuff, and then read the entirety of Puyo Puyo~n's entry in that thing. Believe me when I say EVERYTHING about Doppel will make SO much more sense after you do. Her hatred for Arle AND Satan ESPECIALLY. Heck, because of this, I'm wondering more than ever just how Compile would resolve her character arc, if they'd be able to do it at ALL considering that it feels like Doppel would be at one heck of an impasse.
BTW, all that stuff about Doppel and Arle? It technically carries into Fever 1 and onward. ESPECIALLY if Puyo Quest's new alt. for Doppel is any indication.