I'm going against my better judgment to add my two cents. I just got back into the game (completely missed 4E and a variety of things surrounding 5E's launch got me interested enough to recruit a group of folks who have never played before). I've just purchased the core books and I'm still working my through them. I'm not up to speed on the last eight or ten years of RPG developments (I vaguely remember Pathfinder and 4E being developed as I started checking out). All that is to say that I am not speaking from a position of authority. I'm just making an observation that has probably already been made before.
The release schedule so far has been tied to themes with that theme being emphasized with various ancillary partners. I wasn't around for the dragon emphasis so I can't speak with authority there, but elemental evil has been announced with minis and video game support. Both of those take time to develop and coordinate with other elements (especially any video game synergy). Both of those may have a different season of sales strength before needing the next thing (or to put another way, ancillary products may have a different sales cycle than the RPG books and Wizards may be trying to balancing its various income streams to maximize each stream's profit potential before introducing the next cycle) To me, it looks like Wizards is trying to coordinate the brand to launch products on theme to maximize the impact of advertising/mind share. That may be lurking behind a slower schedule. They aren't interested in launching generic support products at the moment (a spell compendium or such). They want to launch themed products that synergize across the entire brand.
So what are they working on? They're working on developing the next several themes and plotting out support for those themes. I don't know if this happened in 4E, but I know it wasn't like this in 3.X.
As an anecdote, I will add that this worked to get me back in. 5E came out to enough positive reviews from different outlets that it made me think about playing again. Now, that I'm back in, I am sorely tempted to take part in the theme just to be able to have that shared experience. I'm not sure it'll impact the home game that I've started, but I've spent the last few days looking at AL and figuring out if I can fit that into my life. It's also made me want to buy the adventures just so that I can read about the experience. It's also made me remember that I wanted to check out that Neverwinter MMO, but my internet isn't up to snuff for that.
In my opinion, I wouldn't waste my time with the MMO. It's not good to be honest. It is extremely "grindy" and becomes boring really quick.
You are right about the theme, but the problem I see is they are really trying to shove the theme down our thoughts buy almost making you buy their AP's because of the information inside. There is nothing wrong with having a theme, but it shouldn't be getting most of the attention, it should be running along the side with other non themed products because there are lots of people who are either not interested in AL or don't live close enough to a shop that does it.