This is a really interesting point, and I think it's actually a really good point; overall, while I like getting additional books, I remember the point at which I could no longer just toss my 4E books in a duffle bag, because the bag I'd been using to carry "my gaming stuff" was no longer big enough to hold them. Let alone hold them plus any other stuff I might want handy, like dungeon tiles or whatever.
Bloat doesn't have a firm definition. I think it's at the intersection of being unable to find content you want easily (by being uncertain of the location), being unable to easily transport the material, having books that might never be used in play, and the balance of material being affected by power creep.
Give or take...
I would point out, though, I don't think that books like the PHB or various splatbooks are exactly "consumed"; I mean, I keep using them. I do start to be familiar with all the things, so they aren't Interesting And New, but that doesn't mean I use the new books instead, it means that I use the new books too. And that's where we start getting into Too Many Books.
Agreed. Although things do get weird when every option at the table is not from the PHB.
I'm not sure what you mean by "digital content"; for instance, I'd love to be able to buy PDFs of the books. (And I don't think PFO's problems are a result of what they're competing with, so much as of them making a large number of spectacularly bad decisions.)
I'd love PDFs as well.
Digital content is neat, but it does compete with the physical books, which is unfortunate. It gets necessary for games like Pathfinder or 4e, but I don't think it's as needed for 5e. There's just less math and complexity in gaining levels.
I'm not sure it makes sense to ask what specific stuff is missing, because part of the point of new content for an RPG is that it's new to you, so until you have it you don't have any way of identifying it. Before the original AD&D MM2 came out, I couldn't have told you I wanted Modrons, because I'd never heard of them. But they have been a wonderful addition to the game that I've used more than once.
Things I would definitely like would include tons more monsters, some sort of usable psionics rules, more variety of sorcerer bloodlines (I'd like there to be a number of bloodlines comparable to what clerics got for types, say), and much more comprehensive material for things like outer planes and post-20 adventuring. I wouldn't at all mind more of the options that showed up in 4e, such as shamans or warlords.
For what it is worth I want all that stuff (except for the post-20 adventuring stuff - I'd never use that) as well. I find I really could use more mid-level and high-level NPCs as well. That small NPC section of the MM is heavily used in our games, and I could really use a wider variety of those.
I'd love more monsters as well, which could include some more NPCs.
Which is why I backed
Tome of Beasts. So I might be good for monsters in 6 months.
Other than that, I'd like 2-3 more subclasses for most of the classes. Maybe 3-4 for the barbarian, bard, and druid who are a little under loved. But that's far less content than you could squeeze into a single 256-page book, let alone a full-sized PHB2.
Seriously, look at the numbers. Each subclass takes roughly 3/4 of a page, so if there were 4 new subclasses for each class, that'd be around 40 pages. Maybe 50 with art. To almost double the subclasses in the game. Add subraces for each race and you might get 60 or 70 pages. What do you fill the remaining 180-odd pages with? Some spells, some feats, some martial manuevers. Yeah, that'll kill 20 or 30 pages. 150 more to go.
What then? Double the content? Triple? At that point things just become silly...
At that point, the game is pretty much done. WotC could release a single giant splatbook and "finish" the edition in terms of support in a single month. There'd be room for a big all-in-one tonal expansion (like psionics) with classes, subclasses, races, monsters and the like, but not much else would be needed.
Or they could space out that content over three or four years, with small releases that are heavy in flavour (like in the post-Essentials 4e line). Making each book with its content more anticipated and exciting.