DISCLAIMER: No, I don't work for Blizzard neither does my neighboor's girlfriend's cousin. The text below is just some speculative thinking. Sorry for any grammar errors.
2014 (november-december):
- WoD launches as expected.
- Some increase in subscriptions.
- New PvP zone is ok, new raid tier is ok.
- Some imbalances in PvP thanks to new talents/perks.
2015:
- We get 2nd raid tier.
- Some talents are revised because of PvP imbalances.
- Subscriptions fall.
- Blizzard plans to launch another expansion at end year but can't deliver. We end up getting another raid tier, but it feels rushed.
2016:
- Another expansion. Some increase (small) in subscriptions.
- Blizzard says (without using the word 'promise') they will try to deliver a faster expansion next time.
- Talent system gets a small revamp; now there are more talents that change based on spec. There's some rework on spells too, some spells (not much) are shared between different specs. We get new class/specs mechanics, but not many new spells; in some cases (but not all) the amount of spells decrease between now and previous expansion.
- As it was the case with WoD's level 100 talents, there's new talents that change playstyle when selected, which prompts people to talk about "subspecs" or something of the sort.
- More option are added to the game; I have no good guesses about this but the game surely isn't just divided between PvP and PvE. Aside new 'sidegames' like pet battles and garrisons, and stuff like the brawling and proving grounds, there's more new stuff now to do besides 'normal' PvE (dungeons->raids) or 'normal' PvP (arenas and bgs/rbgs). I don't have a good guess about *what* will this new stuff be, but surely there will be *something*. What's important to note here, this isn't just a new 'sidegame', but a new 'path' inside the same game. This either can work during this expansion or in the next one.
- Subscriptions fall either way.
- We get just 2 raid tiers this time. The 2nd tier drags on for some months, but...
2017:
- Blizzard delivers its 'promise' of a faster expansion, with a delay of a couple months.
- Another talent/spells revamp. Now there's even more spells shared between specs, spells exclusive to one spec are mostly attached to talents now.
- A couple talents in a couple classes/specs even change or give more than one spell when selected.
- However, even more base spells are taken away. Also, most CDs are taken, so each spec has just 1-2 major CDs; new raid fights are tuned around this. With fewer CDs, most of the time you're just smashing your rotation, but the rotation can change subtly based on situation (like ST or AoE, freely attacking or defending, in the move or no, and so on).
- Every spec is now a different combination of almost the same base class spells (barring some exceptions and 'exclusive' spells obtained thru talents) with different procs/rotations/playstyles determined by the current spec.
- However, continuing the trend of last expansion, all the new talents (last talent tier) change the spec playstyle, giving new procs/rotations and so on. The term "subspec" is almost official now.
- The number of talent lines gets crunched back to around 6-7 lines.
- Few talents are shared between specs, most of them now change based on selected spec.
- Because of these changes some specs are overpowered in PvP; this is fixed in later patches... well, kinda. Some specs stay overpowered until next expansion.
- 2 good raid tiers delivered within this expansion, with the next expansion delivered either this year's end or next one's first weeks. People now call these "mini-expansions". Subscriptions fall.
2018:
- New expansion (either this year or end of previous one).
- Subscriptions grow a little, or stay about the same. Either way, subscription growth (with new expansions/patches) or fall (with time) are much smaller now, so subscribers begins to stabilize around a number (I have no guess about this number).
- Classes/specs now feel like a general framework, where you have the same base spells for the entire class (let's say around 10-12 spells), where the spec gives you a couple different spells and different procs/'chaining' between base spells, or change base spells in some way.
- Talents are different between specs, with some minor exceptions. Talents now usually change some base spell instead of giving you a new one (with some exceptions), or just change the way you 'chain'/rotate spells, or give/change procs.
- So the end result is that each spec is indeed a general framework with lots of possibilities. However, the total number of spells you have to use in each case (your basic rotation) is less than in previous expansions. You also have fewer situational spells; around 1-2 defensive CDs, just 1 dps CD, and so on. What's more important now is the way you 'chain' your few spells, this will change the end result, either giving more or less dps/heal, or favouring more ST or AoE dps, or giving you defensive buffs or giving your enemy debuffs, all this interweaved in your basic rotation. With some exceptions, this is how things play now.
2019:
- Things go on mostly the same: new mini-expansion, raid tiers, subscriptions either grow or fall but stay around the same number (very small by past standards but stable). WoW now is a very different game but still has it's niche and still has some playerbase.