Expanding a bit from the "themed sourcebooks" concept...
I worked on two of the "Complete X" books in 3E, and they include some work I'm quite proud of. I also contributed to one of the "X Power" books in 4E, though I didn't do any of the mechanics in that one. Point is, though, I worked on both, and they were both relatively good experiences.
And for all that, I do
not want to see these kinds of books in 5E.
Why? Because I want all the new mechanical goodies--classes, subclasses, feats, spells, magic items, monsters--to be
concept-driven. I'm a big proponent of options, but not for their own sake.
What I mean by this, is... I don't want to see a book where the design goal is "Provide new toys for classes X, Y, and Z." I don't want to see a feat--
any feat--created because the book it's in had more space to fill in that sub-chapter.
I want to see lots of new stuff, yes. But I want to see it because someone had a good idea for it, or because it enhances the feel/theme of a new setting or campaign style. A new battlemaster power because it's inspired by something traditional samurai did, appearing in a sourcebook on Asian-themed campaigns? Yes, absolutely. A new battlemaster power because "Hey, we should give the battlemaster more powers"? No.
And BTW, I do realize that this--and my above post--may be impossible given the demands of the market. I know that splitting their focus into too many niche products is part of what killed TSR with 2E. I get that books of player options tend to sell. If that's the reality of the situation, so be it. I still expect to like the edition. But we're talking personal preference, so I don't
have to take market factors into account.
