In Episode 5 of the WoW Forever Podcast, near the end of the hour and a half long episode, Clay Stone dropped some hints about what might be coming in the future in WoW Forever. Here’s the full quote, which you can hear at around the 1h38 minute mark of the podcast:
One of the things we’re trying to do with Forever is to make sure we’re laying the groundwork for the future. There might be some homes, some manors out there that folks have found, that might be something interesting in the future, you might have seen some other instance portals kind of hiding in inconspicuous spots. So, there are things coming in the future, and we’re going to be really excited to share those as we get there!
We have done the digging, and we’re bringing you many areas in WoW Forever that are likely to become content in the future!
Manor Mistmantle, Duskwood
The first location (that was more or less name-dropped by Clay Stone on the podcast) is Manor Mistmantle in Duskwood. Home to the The Legend of Stalvan questline, the place has received a significant upgrade, with a whole new gigantic manor built there!
The Manor itself has no signs of instance portals. Still, the presence of the huge manor itself (as well as the level 60 Mistmantle Prowler that terrorized Alliance players in Duskwood during the Beta more than Stitches ever could) shows that this is likely a location that players will be sent to in the future.
Bael Modan, The Barrens
The dwarven digsite in the Barrens is housing a hidden dungeon portal! The area includes storylines for Horde players, such as a dwarf being morphed into a trogg for being exposed to an artifact for a long time.
In the original WoW timeline, after the Cataclysm, we are warned to stay clear of Bael Modan, as the dwarves have dug something that “should have been kept buried”. It would be interesting to see how such an artifact could be approached in a dungeon format.
Rustmaul Dig Site, Thousand Needles
Another dwarven dig site, this one was abandoned after a battle between Horde and Alliance in the area stirred a nest of silithid creatures, who ended up overtaking the site. This one also has an instance portal hidden behind a wall of planks, which is very easy to miss if you don’t know what you’re looking for.
Farholde Keep, Riverglades
The new Riverglades zone already has one known dungeon, Krol’dok Stronghold. However, you can find a second dungeon portal by the shores of Farholde Keep, the Alliance hub in the zone.
Dwarven Farm, Wetlands
The farm on the top of the mountains in Wetlands is a famous sight for any Alliance player who has even flown to Menethil Harbor. You can now access this area on foot from the Excavation Site: Wetlands dungeon area.
One of the entrances that used to lead to a way under the terrain now houses a dungeon portal! That currently leads nowhere, of course.
Shaper’s Terrace, Un’Goro Crater
Shaper’s Terrace is a new max-level dungeon in Un’Goro Crater. It is currently inaccessible except for the entrance area, but you can Eagle Eye inside the dungeon. Digging far enough, you can find… a raid portal?
This would not be the first time raid portals are hiding inside dungeons. Molten Core’s original entrance is inside Blackrock Depths, and Blackwing Lair’s is inside Upper Blackrock Spire. Having a titan-themed raid would definitely be interesting!
Windshear Crag, Stonetalon Mountains
Although this area does not really hide an instance portal, you can find a new area on the top level of Windshear Crag. Just before the bridge leading to the leaders of the Venture Co. operations in the area, A new area leading to a mine entrance can be found.
As this area does not house an instance portal (and is blocked off for the most part), this could lead to an “outdoor” dungeon area, like Dun Modr in Wetlands or Jintha’Alor in the Hinterlands.
New Avalon, Eastern Plaguelands
The Scarlet Enclave was the last raid to be released in Season of Discovery. Many of its assets, such as its raid entrance portal and the open world area of New Avalon, can be seen in Forever. Funny enough, trying to get into the New Avalon area is not possible, as you will get 
No Man’s Landed back to Light’s Hope Chapel.
These may be just leftovers from Season of Discovery, but it does open up the possibility of seeing Season of Discovery-exclusive content (like Scarlet Enclave, Demon Fall Canyon and the Karazhan Crypts) return in Forever. Noticeably, some changes happened in Deadwind Pass, with the patching of the hole that led you to the Karazhan smiley face.
Gilneas
Gilneas is an interesting one. Although the Gilneas landmass has been on the world map since 2004 Vanilla WoW, in WoW Forever, you can see Gilnean buildings from the shores of Hillsbrad Foothills and Silverpine Forest.
However, getting to Gilneas was very taxing in the Forever Beta. Surrounding Gilneas is a new zone, the Shark-Infested Waters. The name is very fitting, as you will be mercilessly stunned and killed by level 63 elite Sharks the moment you swim into the zone. And if you are patient/stubborn enough to death run through these waters, you will eventually reach the shores of Gilneas… only to then be mercilessly stunned and killed by level 63 Worgen. That is definitely a lot of security! What are they hiding there?
As you can see, from the Forever Beta alone, many areas could lead into future content. Where would you like to go in future patches?