A second manual of monsters is almost surely in the offing. It should be called the Fiend Folio. I just hate the idea of "Whatever Manual 2" books when you have perfectly good, traditional titles you could use. If, for whatever reason Fiend Folio is not owned, rights wise, by WotC, then come up with something else! "Whatever Book 2" is a complete cop-out. And yes, I am aware there was an MM2 in 1e. Wasn't any better then. There are enough synonyms for "Monster" and "Manual" that "2" should never have to appear on any cover ever. Same goes for PHBs and DMGs.
"Crowd-sourcing" (with a page of credits for the lucky bastards) from the DmsGuild is a great idea and would be very much in keeping with the original FF.
I think, however, that regardless of original DmsGuild material, there is no shortage of critters from previous editions (including the MM2 and FF) that could easily fill up a second monster book.
Personally, I'd like to see...
- Animals: pick another 10 to 20 normal/more unusual animals and draw them up.
- Faye: a lot to work with here that hasn't been covered. Leprechauns, Swanmaidens, Nereids (I would support the continued elimination of the generic "nymph" and instead, as with the Dryad, have the tree nymphs, water nymphs, mountain nymphs, etc... as separate [fae] creatures), nixies, redcaps, spriggans, fossegrim, brownies! TONS of fae to choose from. Let's say, easily, they do 10 fae.
- Elementals: just a few (even just 2) beings or creatures for each element: sylph, pech, fire bats, fire newts (always an abundance of fire types, aren't there?), undines ("grown up" water weirds?), some other water-elemental-based humanoid, etc... Really probably don't need more than 4-8 here.
- Dragons: Gem dragons. That's, what? 4 or 5 types? In the MM, a dragon entry is taking 3 pages. There's 12-15 pages right there.
- Humanoids are really broadly and completely covered in the MM. The only things I can think of adding would be the Dark Creepers/Stalkers. But they might not even necessarily be categorized as humanoids...they could be dark fae or extraplanar/shadow beings or something. But, yeah, as far as monstrous humanoids/ye olde "giant class" stuff, they're all there already.
- Lycanthropes/Shapeschangers: Were[hawks/eagles/falcons], Werefox, Selkies...weresharks, seawolves? There's got to be a half dozen people could come up with easily.
- Aberrations: I'm really not familiar enough with these creatures, but seems they've covered a good amount in the MM. Maybe another 1-3 things that would be classified as an aberration.
- Demons/Devils: I think they have these sufficiently covered. Even the "periphery" lower planes creatures (night hags, succubi, nightmares, etc...) are already done. There's really nothing more that needs doing here other than nobles/lordly types. They could, I suppose repeat/reprint all of the demonlord stuff from OotA. Would be nice to have for those of us not necessarily buying all of the adventure paths, though I personally think they are more appropriately included in a Deities & Demigods/rulers of the planes style book.
- Giants: same as Demons/devils. They have the iconic ones all done, and the ogres and ettins and trolls...so "giant-kin" is really pretty finished. We don't need the "corner case/shoehorn" ones (mountain, swamp, fog...really?! Fog?!...giants, etc...) from the old MM2 or FF.
- Other Extraplanar things: I'd like to see some Archons or other celestial beings besides angels and modrons. The Moondog (though I feel like I've seen a write up for these guys before)? Maybe even an aasimar as a monster/npc sidebar?
That's somewhere, including animals, in the 65-70 creatures range. That's a hearty manual of stuff. I'm sure with more than cursory thoughts off the cuff, even just a perusal of the previous edition manuals, we could get that up to 100 easily. How many are in the 5e MM?
Point is...a second monster book is surely coming. Be patient...and pray that it won't be called "Monster Manual 2."