They have a habit of announcing the new expacs a month of few after the end raid/end story cinematic, because after the main end-game raid, there really isn't much for players to do (for those that were playing it the whole time, those like me playing catch up will have more to do), so Blizz would usually hold a con that yr and announce the new expac, but since that's not happening atm, I'm not entirely sure when they'll even announce it.
They could just shadow drop a trailer or sneak peak cinematic, but for a game like WoW, that's rather small (Even SE does more for marketing it's new expacs these days than Blizz does).
I'm a bit excited and disappointed that Blizz is going to go with the Dragon Isles, mostly because we knew since Cata that the dragon aspects lost all their immortality and left the "future of azeroth" to us, the players, so having us fight Dragon aspect battles for them on the Dragon isles like we did for Khadgar during WoD/Legion leaves me a bit frumpy.
I guess this is them just dragging things out before Azeroth somehow takes some spiritual or physical avatar form, because I know the end-game for WoW is us, the light vs the Void, and then that's the ultimate end of WoW's story (There really isn't anywhere to take it afterwards, because we've done the whole faction vs faction, faction vs the light, faction vs the dark, faction vs the new unheard of bad guys etc).
I just hope that with the new expac, that we actually get to go out and do more meaningful quests, instead of being glued to an anti-social mission table, that's mostly composed of fetch quests or timed ones, all to gather small drops of resources (seriously, I'm really sick of mission tables and resource gathering and they've been doing this kind of thing since the first Vanilla raid of Ahn'Qiraj).
Yeah I feel the next expansion really needs to pull people back into it. The amount of timegating in SL to appease the hardcore was pretty bad and it put a lot of players off, with very little content outside of raids and singular area's to farm. And those mission tables are awful idd. I think I quit Retail during during 9.1 as I don't really care for raids and it was too much work/grind to actually play the game for the story.
Dragon Isles being the centre does seem promising. Though long term players want to see the game story progression at a faster pace. Like its been a long time coming to see the battle for battle against the Void and it kinda feels like they're dragging it on a bit.