All I heard is "I want guild boats." Additionally, the Burning Legion is an integral part of WC lore since Warcraft: Orcs vs. Humans; the legion has since been present in some form or another since then (i.e. Medivh's possession by Sargeras and Gul'dan's use of that relationship to open the original dark portal). You can let me know what the south seas has on that any time you're ready.
On another topic, there's plenty of untouched lore out there. We don't know what happened to Azshara (other than her being turned into a naga). We know the Emerald Dream still has some vestige of corruption in the Rift of Aln (rumoured to be the Old God, N'zoth). We know that the Titan Pantheon is out there still and that they know what has happened to Azeroth since their departure; there are even some reasons to speculate that they may consider Azeroth a failed experiment and may come attempt to wipe it out at some point in the future - a la Algalon - due to "systemic corruption" i.e. old gods. We don't know what happened to the Titan Watcher, Tyr, who has mysteriously vanished from the face of Azeroth. There's even more, that's just me shooting from the top of my head. Bottom line, there's so much more out there than "pirate adventures on the high seas!!".
Do I think we should visit the South Seas in a future expansion? Heavens yes. Should it be the FOCUS of the expansion, absolutely not.