FINAL FANTASY X-3: Definitely, Maybe

[Sephiroth]

Sin comes back so there is no reason why he couldn't. Both have been sent by Yuna. Maybe he is even the reason why Sin comes back as he already is Sin, for example and has managed to return. I would love a story with him again. I missed fighting him in the Via Infinito. Kinoc, Jyscal, Yunalesca and Zejon but not Seymour.

This would be one epic final battle with a new ultimate form for him.

[Sephiroth]

Apparently there are no definite plans - for now (artwork for Tidus, Yuna, Audio Drama and Novel are pretty obvious if you ask me but better not announcing something too soon):

"Seit wir uns wieder mit Final Fantasy X und X-2 beschäftigten, welche vor über 10 Jahren veröffentlicht worden sind, führten der Szenarienschreiber Kazushige Nojima und Charakterdesigner Tetsuya Nomura eine Diskussion und dachten, das das HD Remaster eine gute Möglichkeit für sie wäre, diese zusätzliche Geschichte anzufügen. Aber die Einbeziehung der Audio Episode hatte nichts damit zu tun, das wir Pläne für ein Final Fantasy X-3 haben."

Translation:

"Since we've been busy with Final Fantasy X and X-2, both released over 10 years ago, both Scenario Writer Kazushige Nojima and Character Designer Tetsuya Nomura held a discussion and thought about the HD Remaster giving a good opportunity to add this extra story. However the inclusion of the Audio Episode did not have anything to do with us having plans about a Final Fantasy X-3."

Source: http://final-fantasy.de/content.php/...-HD-in-Planung