Naoki Yoshida has mentioned that he has two primary reasons for bringing in guest creators to Final Fantasy XIV: (1) It has a big promotional element that can be used to market new content and (2) to bring in new design ideas outside of the usual development team which would help deliver a new type of experience for players. Judging by development interviews that I've read, it seems like bringing guest creators when possible is something they would like to do looking forward.
Here are some of the guests, both inside and outside of Square Enix, that has developed content for the game:
I would personally like to see Hironobu Sakaguchi, the creator of Final Fantasy, to be a guest creator to make new original content for the game. It would be interesting to see what kind of original content the father of Final Fantasy could make for a modern Final Fantasy game that he has never worked on before.Yasumi Matsuno - Return to Ivalice
Notable Works: Tactics Ogre, Final Fantasy Tactics, Final Fantasy XII
Role: Scenario Writer
Keita Amemiya - Return to Ivalice
Notable Works: GARO, Kamen Rider series
Role: Boss Designer
Tetsuya Nomura - Eden
Notable Works: Final Fantasy VII-VIII, X, XIII, XV, Kingdom Hearts series
Role: Character and Boss Designer
Yoko Taro - YoRHa: Dark Apocalypse
Notable Works: Drakengard series, NieR, NieR: Automata
Role: Scenario Writer
Yosuke Saito - YoRHa: Dark Apocalypse
Notable Works: Dragon Quest X-XI, NieR, NieR: Automata
Role: Supervisor
For art, I would like to see Hideo Minaba and CyDesignation come in and do some designs for FFXIV. Hideo Minaba previously worked on Final Fantasy Tactics, Final Fantasy IX, and Final Fantasy XII. He's currently the artist responsible for the hit series Granblue Fantasy. If anything, I would like to see CyDesignation handle the art direction for future Final Fantasy titles such as Final Fantasy XVI since it's practically a superteam consisting former Final Fantasy artists including Akihiko Yoshida.
Tetsuya Takahashi of Monolith Soft would also be great as either guest artist or scenario writer. He's known for the Xeno series such as Xenogears, Xenosaga, and Xenoblade. He also worked as art director for Final Fantasy VI and Chrono Trigger as well as concept art for Final Fantasy VII. Since Square Enix allowed Tetsuya Nomura to design a character for Xenoblade Chronicles, it would be cool if Tetsuya Takahashi could return the favor and design something for Final Fantasy XIV. Since he's a big fan of mechs, it would be interesting to see him contribute something for PvP such as Rival Wings or something completely new.
Who would you guys like as a guest creator for the game?