When I come to think about PHB III and IV, there are basicly two possible ways that it could be (probably more ways, but mainly these two):
1) Theme based.
Let's assume that Ki stands for Martial the same way Elemental stands for Arcane (and vice-versa). So that with Ki and Elemental you could base a whole Asian/Oriental campaign/setting, with ki getting monks, ninjas, sohei and whatnots and then Elemental getting the Shugenja (controller) some air/water healing dude (Leader), some fire-based dude (Striker) ad some Earth-based ironskinned dude (defender)... well, I dunno about these examples, but I think you got the idea.
That way they could simply dedicate a whole book for that kind of stuff:
Let's say,
PHB IV :
4 Ki classes
4 Elemental Classes
Then on the earlier book (PHB 3) we'd get another 4/4 split.
PHB III:
4 Psionic Classes
4 Shadow Classes
They could give a much higher emphasys on the setting on those years, in 2009 probably going along with, say, Dark Sun, and on 2010 some oriental setting (either new or another attempt at Rokugan).
Just because PHB I and PHB II are following a 4/2/2 "method" it doesn't have to mean future books must follow the same format, even more considering they had to cover lots of ground on the beginning, and missing some key elements would had been bad. (ie: Fighter, Cleric, Rogue, Wizard; 4 Classes, 3 Power Sources, remove one and you're screwed).
It doesn't necessarily need to be that way for future products, but see below.
2) "Distribution" based.
Following earlier PHB books, they would still split them (the classes) under the 4/2/2 as it would give a more "incomplete" feeling and thus would sell better, as you'd end up with the feeling that you'll want the next book to get the classes that got left out.
On the same note, they can't for instance give us everything we're missing from earlier editions in a single book. Monk, Illusionist, Necromancer +Psionics, if they bring all that in a single book (PHB 3), there won't be anything "important" on the next book that's significant enough for some people to get it. Even though the idea of having a book containing 8 new and never before seen classes would be awesome for some people, for some other people it would be the complete opposite, which means, they wouldn't buy a book if there's nothing in there that they already identify with.
For that reason, I think that if it follows that method we'd see something like this (although it could work some other way):
Shadow (4)
Ki (2)
Elemental (2)
Psionics (4)
Ki (2)
Elemental (2)
Pick these two distributions any order you want for PHB 3 and 4. (And also, Shadow and Elemental being interchangeable).
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Which way do I think it's best?
Actually both ways have problems.
See, when I say that "some people might not like the book for it doesn't have something 'important' for them" in method (1) it's much easier for that to happen. Myself, for instance, wouldn't buy a PHB fully centered on Oriental classes, as I personally have no interest on it, and the way I divided it up, PHB 4 would be fully "Oriental".
Method (2) gives the feeling of "incomplete" as mentioned above, which I don't like either.
Either way, I'm getting the impression that my chances of getting any PHB after PHB II are getting considerably diminished as time goes by. In the end, naturally, only time will tell when I get to see what's inside, and then decide about it.
Anyways, this is just some random speculation from my part, sorry if I made you read through all of this
I don't think they'll be ranged - or, at least, I don't think they'll be exclusively ranged. That is, avengers could have two builds, one for up close and personal melee avenging, the other for avenging from a distance, behind the defender.
