Fascinating. I think one prediction can be safely made about 5E: whatever it is, whenever it's out, its primary design goal will to be recognisably D&D. To as many people as possible.
4E went the other way. The 4E design team too probed how far they could go in making a good game without going too far into the area of 'that's not D&D anymore' (see Races&Classes designer diaries). But their point was that on occasion they were happy to sacrifice 'sacred cows', both mechanically and story-wise (something Mearls' L&L columns haven't even
touched on*), if it made for a better game. That's exactly what they went for: produce a mechanically superior game, even if it went against D&D's history, mechanically and otherwise.**
This sort of trade off will be anathema to 5E. 5E would rather harvest recognisability - because that translates into recognition and acceptance - than a superior game engine.
Mind you, these two things need not always pull in opposite directions; it's just when they do, you got to make up your mind where your priorities are.
This suggested inversion of priorities would also fit with
making the D&D fanbase less divided a primary goal, something close to the surface in Mearls' first Enworld posting when being made D&D Group Manager in 2010:
I'm acutely aware of the pressure of the position, the expectations, and the current atmosphere among D&D fans.
That whole
post is worth rereading. Replace his references to 2E and 3E with (respectively) 4E and 5E, and you got the perfect picture of the reclaimed fanboys they're shooting for.
* What if we need the Great Wheel, five types of humanoids as low level foes, and so on for it to be D&D? What if D&D today, as opposed to in 1976, is a cultural icon with a
history, and that you better not ditch 90% of that history in revamping the game for a contemporary audience?
** Here's how Mearls
describes the mood at WotC R&D when 4E PHB 1 hit the shelves in June 2008:
With the PH 1, there was a lot of stark terror that everyone would hate the new game. That tempered everything - there was a lot more tension and waiting.