Will Final Fantasy ever make a "Dark Side"?

[Rosenstrauch]

True, Raktapaksa is an oddity. (Is it a reference to another FF thing or is it completely original?)

It's half a reference, and half original. "Raktapaksa" means "Red Wings" and is another name for the mythological entity "Garuda". Meaning Ifrit and Garuda fused to make... Garuda, but red.

Also the business with Ryne-Shiva was definitely odd. I think they just wanted to work some kind of dramatic excuse for Ryne to lose control and have Gaia come and save her, never mind the plot logic of doing so.

Oh, don't get me wrong: I'm rather happy we didn't get another Ysayle-Shiva. Though I've got my own gripes with how Ryne-Shiva played out.
The impression I got from it was that Ryne really wanted to be Shiva and Urianger, rather than be the voice of reason, covered for her by lying about the necessity of it. Considering how she lost control of the primal instantly, I was hoping to call them both out on endangering the world to indulge her. But the opportunity never came to pass—the closest it got was the Warrior of Light getting the choice to brush it off with a joke.
I am very confident that's not what the intended takeaway was. We were definitely supposed to take Urianger's jargon as a serious explanation. But I just can't do it.

[Iscah]

The impression I got from it was that Ryne really wanted to be Shiva and Urianger, rather than be the voice of reason, covered for her with by lying about the necessity of it.

Reading through it again (since I hadn't reviewed it since I first played it) I don't think he's covering for her at all. He's saying it's a bad idea even if he can see some degree of logic to copying the method of summoning, and I think he's maybe trusting to Ryne's understanding of her grasp on Eden's power and how she needs to handle it. He certainly doesn't seem enthusiastic about it, he just can't argue against it.

(Also, to be fair, we have never actually killed a summoning-vessel human by defeating their primal form. Ysayle, Thordan and Yotsuyu all returned to human form and mind - and Ysayle in particular at least three times over without dying: our first fight, the EX trial which is a canon re-summoning, and against Ravana.)

[Rosenstrauch]

Yeah, it seems like he did speak against it more than I remember him doing—his arguing its merits is still all kinds of daft, but he's not as complicit as I recall. Looking at it, I think I must've been attributing Ryne's arguments to him. I still want to call her out on it, but I feel like the opportunity might've passed for that.