Hmm I remember thinking MoP might be real but I guess I don't remember WoD well enough. I do think that when the pre Blizzcon leak happened it was very clear.

Memory serves, MoP's title itself leaked a while before the reveal, right? I remember a lot of arguing over it as some claimed it "didn't sound like an expansion" which always made me laugh because the idea Blizzard has rules about it is hilarious.
People said the same thing about Cataclysm, too.
In regards to the early Iron Horde leak;
Iron Horde and the next WoW expansion
Hence the "even when it's the expected theme/villain".
A lot of people were expecting a 'Burning Legion' expansion coming down the pipeline. No one was expecting a Broken Isles expansion with Extra-Night Night Elves, the Emerald Nightmare, Mountain Tauren and Vrykul directly serving a titan watcher. When we got the picture leak that had Illidan, the Illidari, Syvlanas, Ashbringer, the Silverhand, Naga and Wardens, people thought it was ludicrous.
People, even the week leading up to BfA announcement and when we got the Lordaeron symbol leak, thought BfA was going to be a Lordaeron 2.0 revamp/Council of Glades sort of thing. When the entire Lordaeron bit was just a prepatch thing. Even with the KT armor set people weren't expecting a dual continent KT/Zandalar world-spanning expansion about a war for crystallized titan blood that progresses into going to Nazjatar and Ny'alotha which is actually a dream state of reality not a physical plane or place.
SL and Dragon Isles were only "accurate" in the sense that both those locations were so completely vague prior to the two expansions, that there was nothing to actually 'see coming' besides the name of the place.
My point is that the expansion has pretty much never actually been what was assumed. It could very well be Avaloren... and Avaloren could be the Titan designation for one of the two moons of Azeroth, and 11.0 could be some sort of BC-esque extremely exotic environment fantasy expansion. It could be a revamp but at the end of 10.2.5 or 10.3 the player characters and some NPCs get stasis locked and 11.0 takes place 250 years in the future in a totally different looking Azeroth. It could be a void expansion but it's not K'aresh, that's just 11.3 or the final raid, and the continent is Azjol'Nerub under the unified Nerubian/Naga forces as a new void empire.
So the idea that any given expansion is "clearly" going to be some specific idea is sort of self-contradictory, every guess that isn't just an outright leak is just a shot in the dark hoping to graze part of some nebulous whole.

If I recall correctly, during pre-WoD leak season is when Heroes of the Storm was trademarked and we knew nothing about it apart from the name. Some of the folk here were predicting that could have been the WoW expansion, with theories relating to again, Outland, as well as potentially an elemental planes expansion. It was a very long time ago though so my memory of it is a bit sketchy. Just vividly remember discussion surrounding the HotS name.

If it's not some nebulous "multi-theme on the box" expansion, I can see it being Scarlets as the main figures for a Light expansion. Even if they were mooks in the past they are hugely popular and the build-up is there. New Avalon/Avaloren as well.
Nobody is really expecting the Scarlets in a large capacity either. It's usually Light/Yrel/Turalyon.... and Scarlets too, or something similar.

Next 6 month sub mount probably will be some kind of tease.
Have to agree. We will probably have a good idea on some elements but at the end of the day there will be far more that will be completely different to speculation.
Heck look at Evokers. We DID speculate about a race that is also a class and is draconic themed but nothing like what was delivered.



The story behind the WoD leak (I may be misremembering some things.)
I was an active member of Scrolls of Lore, and another active member worked at blizzard as customer service or something. A couple weeks before blizzcon, it was "all hands on deck", so most of the company got a glimpse of what was being announced.
He wrote up the "leak" in an email and sent it to me, Red shirt guy, and maybe 5 to 10 others that he felt were trustworthy. One guy (wasn't me) copy-pasted it to Mmo champ and SoL.
Hoping to protect the leakers identity so that he didn't get fired, RSG asked one of the mods of SoL to remove the post. That mod was an intern at blizz and simped hard for the dev team, so instead of helping the guy, he ratted him out which got the leaker fired, sent back to his home country, and aided in the eventual death of SoL.

If I recall correctly, these were the main leaks/theories discussed the last few leak seasons.
Pre-WoD:
- The actual WoD leak.
- I vaguely remember some discussion around the WoD name being accurate but the content not, and that the Warlords would be Legion commanders on a new part of Outland.
- The Dark Below
- Heroes of the Storm
- Corgis Unleashed. Rightfully seen as a meme by most but was some genuine discussion surrounding Corgis being the name of a Void Lord. Yes, that happened.
Pre-Legion:
- Azshara/Naga centric expansion, mainly South Seas. Purely because of the shipyard.
- Council of Glades. I remember at the time of this being “leaked”, the majority of folk were happy to assume this would be the one.
- Ogre island south of Draenor was discussed, with Legion influence. Corrupting the Ogres instead of the orcs.
Pre-BfA
- World revamp because of Sargeras sword.
- Kul’Tiras because of the data mined items.
- An expansion focused on reclaiming Lordaeron.
Pre-SL:
- Dragon Isles cause of Wrathion mentions and Island Expeditions
- The fake Shadowlands map leak that Asmongold and Pyromancer were championing.
- Wrath of the Bolvar
- Wrath of the Lich Queen (Sylvanas takes the crown, revamped Northrend)
- The Undead in SW harbour.