Several legendary items were given through the Pandaria questchain. Granted, two of them were gem upgrades. But there's certainly room for them to add more content - they haven't flat out said this will be the final raid zone, and Archimonde as the expansion's final boss would be a little strange. If we get a tier beyond Hellfire, it's possible that we could get another questchain for another legendary item.
Again, what would we be killing? We are only here still because khadgar wants to destroy gul'dan before he gets out of hand. The hellfire citadel is under his control. He will obviously be a boss. So if the reason we are still here is killed, as well as the ones giving him orders, what is left?
I will give you that fact that they could add a gem or something, but that doesn't answer the question of whats left.
I hope new expansion, but Guldan and Grom are not in the raid. What could that mean?
Pretty sure Metzen said at Blizzcon that WoD is part 1 of a multi-part story arc. This expansion would not be a stand alone story like previous expansions. Grom and Gul'dan do not have to be resolved in 6.2.
While we can't know for sure, I think most things point towards 6.2 being the final patch, Hellfire being the final raid, and the next expansion announcement being right around the corner. I'd say Gamescom for the announcement, or an event arranged around then by Blizzard.
They very clearly said that if things go well on the next expansion's development (back in August, when they also said they already had zones in production for it), then we wouldn't need multiple raid tiers. Since everything we've seen from 6.2 thus far points towards it concluding all the little plots and tying the knots together, it is very possible that things are indeed going well with the next expansion.
I have a feeling it'll go a little like this:
Gul'dan will escape from us, he'll end up making it to the Tomb of Sargeras. Which will then lead to the South Seas Expansion next and after that expansion we'll take the fight to Argus.
So far, Blizzard hasn't really given me any evidence to suggest that they'll be getting their next expansion out in a hurry. Perhaps they're holding their cards close to their chest in order to keep us interested in the current expansion... but until they say otherwise my bet is that they won't have it out until sometime next year.
Unless Gul'dan is a hidden mythic-only boss, there's not really anything to suggest he's a boss in Hellfire Citadel, as we already have the complete list of 13 bosses. It's entirely possible him, Grommash, or anyone else could die in the hidden sound files, but neither of them appear to be bosses themselves.
That said, I still believe you're right. Neither of them have to die for the expansion to end, and I don't really see where we could possibly go from here in this expansion.
Of course 6.2 is not REALLY end of xpac.
A 13-boss raid clearly needs 14 months to be properly appreciated.
Of course it'll be announced at Blizzcon, not a day sooner. Been this way for a few years anyway.
I doubt the expansion will be shorter than others. From Wotlk and forward the last patch has lasted about a year till the next expansion. So once we get the sinal patch just add a year and then should be about your next expansion.
Expansion is never going to happen before winter holidays 2016, stop getting your hopes up...
The next expac will be released on the same day the movie debuts, it's obvious...
The next expac will take us to AU lordaeron on the second war with the same heroes portrayed in the movie, to get new players in....