I would say that 5E is not particularly modular, about average as an RPG. The thing is that it was touted as being especially modular for people like me who were interested in a different play style than it eventually embraced. That never happened. That's a fail from my perspective.
I also expected more from 5e in terms of being 'modular,' but I think that was just because of the choice of word. Modular implies, to me, that the modules would be seamlessly interchangeable, which would have been very hard to pull off, indeed. Rather, it seems they just meant that there would be a number of prefab variants that you could work with, as well as the usual freedom to house-rule the game as needed. So, sure, maybe 'modular' was over-sold.
OTOH, I think they undersold-just how DM-Empowering 5e was going to be, with it's rulings-over-rules ideal, you don't really need extensive house rules or even 'modules' to get what you want out of the game. From the DM's perspective, that is, of course.
As for additional options from WotC: this is, for all practical terms, not going to happen. There has been a small amount of material released since launch that's amounted to about 20 pages or so. And we're not going to get much more any time soon, since the staff have been reduced to a skeleton crew.
So far, we haven't seen a lot of new stuff in print. We have seen more than a few things in UA, though, including the Mystic Class. Those things are supposed to be in a sort of ongoing playtest status, so we may see them in print, someday.
Now before I sound too negative (I know, too late!) that one game I'm playing in will be a 5E adventure (specifically the new Ravenloft path). I'm playing it because I have one of the best GMs in the world (seriously: Chris Perkins could learn from him) and because I love Ravenloft. I'm expecting to have a blast for the game. The thing is that it will be despite the edition rather than because of it.
I just ran the opening event, The Death House, yesterday, and it was fairly successful. I had signed up for a 4-hr slot, and it wasn't enough time to finish the run - other tables were still going, but I had plans for late that evening. Most of my players really enjoyed it, the ones that didn't so much, it was because of the venue, which gets a lot of background noise from the other tables (we all had the AL-max of 7 players). I think it promises to be a particularly good season.
Now I know that many people don't agree with me (of course!) but this is really what 5E got wrong for me. So, I'm still with D&D, just frustrated.
I get it, but maybe there's still reason to feel hopeful.