The quest for Siren avatar was initially planned during the development of Seekers of Adoulin but was canceled due to developmental reasons. (Had it been implemented then, someone else would've been in charge, and the scenario would've been different too.) I thought it was such a shame considering her animations and effects were all ready to go, so when we shifted to our current development structure, I made a self-centered request to work on the quest and received approval. That said, it had to be done in the midst of working on other tasks, so it took me two years before it could be implemented.
First and foremost, I was meticulous about how adventurers would go about forming a pact with an avatar following the events of Rhapsodies of Vana'diel, so I brought that up with other planning staff who were well-versed in the lore to write the scenario. I also paid close attention to Siren's Blood Pact ability, and I carefully considered her traits and possible purposes in my suggestion to the staff member in charge of jobs.
The Blood Pact ability's parameters were solidified about two years prior to the quest's implementation, though that was partially due to how long the process had taken. Both Siren avatar's scenario and Blood Pact ability seemed well-received by our players, filling me with joy, relief, and tremendous gratitude to have been granted the opportunity to handle this project. (Takashi Hisagi, Planner)

The scenario writer for Rhapsodies of Vana'diel had warned that Siren would be too complicated to implement from a lore perspective. Years later, however, she was implemented as the culmination of passion that went into the lore research, thorough backstory writing, and adjustments.
There were barely any hints leading up to her introduction prior to implementation, so we were also testing the waters to see if we could add more story in future version updates. As for the results of that test, well... the implementation of The Voracious Resurgence can attest to that. (Yoji Fujito, Director)

Rhapsodies of Vana'diel was supposed to be our last major story mission, so imagine my surprise when I heard we were planning a new story! However, I was feeling rather content with the stories I'd written up until then. So instead, I had some goofy scenario ideas like sending adventurers on an isekai ("alternate world") adventure or turning them into a demon lord, but there was obviously no way those would ever be approved…
Then I thought, "Well, I've had my fun with Yagudo, but there's still beastmen I haven't messed around with." (Incidentally, Bijoux, the apkallu elder, originally had even goofier dialogue but it was edited out.) I ended up becoming very fond of the beastmen, since I not only wrote their dialogue, but also created a huge number of their cutscenes with my own two hands.
…And now that I think about it, my goofy ideas ended up getting tweaked and implemented as the Galkan reincarnation story and the beastmen's... ahem. Well, I hope you'll look forward to the rest of the story to come! (Yaeko Sato, Planner)

It all began from Producer Matsui's instructions to create a prologue of sorts where we'd reflect on past episodes of the scenario. We held a team discussion during the storyboard process to decide what narratives to include, and my two must-haves were the stories for Galkan reincarnation and the Maelstrom Mage, which ended up generating a massive amount of work. (Sorry...) (Yoji Fujito, Director)