The problem with subclasses is they are merely additive. You couldnt do a psion as a subclass, or a worthwhile artificer, since you only have a few abilities to work with, unless they also replace existing base class features. A support warlord cant be bolted on the fighter, because it's core chassis is a multiattacking damage machine.
You absolutely can (and I have) in all of the proposed instances. You [and others] just might not like it/thing it's "enough" of what they want/expect...and, granted, certain archetypes might be easier to construct than others. But you absolutely can do it. For the most part, subclasses give you 5-6 different additional features of design space.
Anywho, I would put money on, a minimum of, the following:
The New Ranger(s) and subclasses is a minimum of 4-6 pages.
The Mystic and subclasses is a minimum of 4-6 pages.
General Psionics combat, adventuring, etc... 4-6 pages.
The Water/Sailing adventuring, naval combat stuff, and character options if an easy 5-10 pages.
We can extrapolate/presume Air/Flying adventuring, aerial combat stuff, and character options for another 5 pages (probably reprinting the aaracokra, et al from the elemental book).
Maybe other "unusual terrains" adventuring, combat stuff, character options (deserts, arctic, etc...) 2-4 pages each.
Added clerical domains: minimum 3 pages.
Added subclasses for other classes, 1-3 pages each.
Added spells and magic items, 20+ pages easily.
Added races and subraces 2-6 pages.
We can assume at least some fan-clamoring will be catered to, so Eberron, Dragonlance, somewhere else (cuz you have to do 3) specific classes, races, and setting specifics is an easy 5 pages, each.
Outer planes/cosmological/Planar specifics...obviously with heavy doses of Planescape-setting and character option shmutz. All told, an [additional/expanded from the DMG] "planar" section could easily be 5-10 pages.
Overly Detailed "added complexity" subsystems for everything from mass combat to going to the bathroom will likely take up the rest/another 50 pages, easily.
I think that's plenty of material to hit the 150 or so page mark for a "major mechanical expansion."