Blizzard confirmed today on the State of Azeroth stream, as a part of Blizzard's 35 Year Anniversary, an updated roadmap for Mists of Pandaria Classic.
Escalation, the next phase, is due to arrive in just a "few" weeks. This will include a new quest campaign, the barrens world event, challenge cards and bosses in brawler's guide, and a handful of new scenarios. More legendary quests will also be added, which you'll want to dive into as soon as possible.

The big news though, well maybe, is that Siege of Orgrimmar is confirmed for Summer 2026, which includes the Timeless Isle, a new PVP season, and new celestial dungeon difficulties. But, what they didn't show us is more telling of what is to come... maybe?
Blizzard has showcased BlizzCon prominently on the roadmap, starting September 12, with nothing after that but they do include "Autumn" afterwards. It is our expectation that Blizzard will be announcing the future plans for World of Warcraft Classic at this year's BlizzCon.
Comments
I must say, I hope that they don't announce Warlords of Draenor. I know that it's still likely to happen, and if they do it, they better make it very accelerated because it is a bore of an expansion. I don't even know if I'd want to see them move into Legion to be honest. We just had Legion remix which was a couple of weeks of fun, but how "Classic" is Legion truly? Legion is the basis for a lot of the modern systems we have in World of Warcraft today, and because of that doesn't feel quite Classic. Warlords feels like such a great cutoff point because Mists of Pandaria is definitely Classic if Cataclysm is considered Classic.
Realistically though, I suspect we will see all of World of Warcraft end up in the Classic timeline and that eventually the two games will converge and there will be an opportunity for Blizzard to merge everyone into mainline WoW which will be amazing for people who missed out on certain mounts "back in the day" that were able to obtain them during the classic timeline. They can keep doing things like the anniversary edition servers to give people a fresh, more classic experience since it's clear people crave that still.
It just makes sense that Blizzard makes sure each of their expansions are available to be played on the most recent, modern WoW platform, and they are getting damn close to having all the truly old stuff up there. I wouldn't be surprised if Warlords and even Legion's games are already playable on the new engine considering those games are where we see a lot of today's WoW tech still.