Hiya.
MM2, easy. 5th edition hasn't been out anywhere near long enough to exhaust the current PHB material. I may be ready for a PHB2 in, say, three to five years. Even then I don't think a PHB2 would be needed...I'd much rather see a sort of "compilation of the best of the best [or at least most interesting]" things that have come out for 5e over a few years. Sort of like an "Unearthed Arcana" that we had for 1e AD&D...a few new class arch-types (say, one or two for each core class), maybe a few new feats (no more than a dozen, please), another dozen or two spells total (all classes total...not 'each class'), some optional modules for things like healing, building/boat construction, skill training, weapons/armor of different materials/sizes, tables for the DM to use for inspiration or "on the fly gaming"...that kind of thing.
If/when a 5e PHB2 comes out, what I definitely do not want to see is a basic reprint of the PHB...but with new classes, spells, feats, skills, etc. Right now, 5e has the backing of many, many "old school", "new school", and outright "newbie" players. I believe that is because it is a quick and easy system, with...and this is the most important part...an obvious slant of game-empowerment being towards the DM and NOT the player. As soon as WotC starts to take 5e down the path of "Well, we have more players, so lets make stuff for them before we make stuff for the DM"...well, that's likely the time I stop buying 5e stuff. I get a feeling that I'm not alone in this regard. I guess the test will be what WotC wants more of...solid, long-term 5e enthusiasts, or vaporous, flash-in-the-pan "it's just a game so I'll drop it as soon as the next kewl new thing comes out" types...
^_^
Paul L. Ming