Sounds delicious.Cookie Gnomes with the power of special ingredients like dryad's needles, cocoberries and unicorn sprinkles.
So a few things I'd suggest that would be cool to see.
1. Dragons
So I do know that FToD will be coming out in October, So which dragons to include will depend on which ones are released in that book. With that in mind however, there are quite a few dragons you could include in MME4. You could add the different age categories of Metallic Dragons that you have included in the previous MME books. You could also include new Metallic Dragons like Steel, Iron, Adamantine, Mithril, and so on. You could instead add Catastrophic Dragons, the 4e Elemental Dragons. They had a really cool mechanic where instead of having a breath weapon, they have aura's that expand and then explode in elemental energy, on top of other passive effects. You also have other Planar Dragons from the different outer planes from Astral Dragons to Rust Dragons, Radiant Dragons and Styx Dragons. You could also expand on some lesser dragons like Linnorms, or add some more powerful Dragonspawn.
Obviously I wouldn't expect all of these but they are some ideas for the dragon section of this book.
2. Mid Level Legendary Humanoids
I really like the low level legendary humanoids that you added to MME3, and I think it would be cool to see a similar concept for mid level humanoid monsters (CR 11-20). This would give people some good options for higher level humanoid encounters for players to tackle with. You could also mix in some other humanoid races, whether they are the more common and popular races (Elves, Dwarves, Halflings, Gnomes, Humans, Tielfings, Dragonborns, Genasi, ect), or less common races (Aasimar, Kenku, Thri-kreen, Warforged, Shifters, Lizardfolk, ect). There are so many options for humanoids.
3. More High CR Creatures
So similarly to my sentiments with more dragons, I wouldn't be expecting a monster manual with nothing but CR 23+ creatures. But I would love to see at least a handful of really high CR creatures that could be used as the big bad or epic end boss battle of a campaign. An Ancient Aboleth that has been hiding under the waves since the beginning of the Prime World's creation. Elder Titans that were so powerful even the gods who created them feared their might. Powerful Primordial Lords trapped and waiting to unleash ruin against the gods who imprisoned them. Elder Primal Spirits that guard the natural world from all Outsiders and their machinations. Liches that have survived eons, gathering all manner of arcane knowledge that has though lost to the ages. There as so many possibilities.
It sort of all comes down to what WotC announces for 2022 - we know we're getting some settings, so some suggestions might be made redundant depending on what they choose to do, especially if they do Planescape. But here are some suggestions.
More extra-planar and elemental planar creatures (like I said potentially redundant if Planescape is announced). Specific requests would be the hierarch modrons, the demodands/gehreleths, other sizes categories for the paraelmentals, the quasielementals (although the quasi-elemental planes have been defunct since 2e, they can just be from parts of the elemental planes that are influenced by the Positive/Negative energy planes), and the xag-ya/xeg-yi. Also the remaining Archdevils and Demon Lords (there are a few major ones of the latter that are still not statted up, like Pazuzu and Dagon) and their aspects.
Some FR-specific creatures: gibberlings (which I am surprised they haven't done yet), nishruu. malaugrym, sharn, and phaerimm.
I'm sure I'll come up with more, but those are the ones off the top of my head right now...
I'd like to add to #2 that adding High Level Legendary Humanoids would be nice, too. Like, give me a CR 23 Archmage.So a few things I'd suggest that would be cool to see.
1. Dragons
So I do know that FToD will be coming out in October, So which dragons to include will depend on which ones are released in that book. With that in mind however, there are quite a few dragons you could include in MME4. You could add the different age categories of Metallic Dragons that you have included in the previous MME books. You could also include new Metallic Dragons like Steel, Iron, Adamantine, Mithril, and so on. You could instead add Catastrophic Dragons, the 4e Elemental Dragons. They had a really cool mechanic where instead of having a breath weapon, they have aura's that expand and then explode in elemental energy, on top of other passive effects. You also have other Planar Dragons from the different outer planes from Astral Dragons to Rust Dragons, Radiant Dragons and Styx Dragons. You could also expand on some lesser dragons like Linnorms, or add some more powerful Dragonspawn.
Obviously I wouldn't expect all of these but they are some ideas for the dragon section of this book.
2. Mid Level Legendary Humanoids
I really like the low level legendary humanoids that you added to MME3, and I think it would be cool to see a similar concept for mid level humanoid monsters (CR 11-20). This would give people some good options for higher level humanoid encounters for players to tackle with. You could also mix in some other humanoid races, whether they are the more common and popular races (Elves, Dwarves, Halflings, Gnomes, Humans, Tielfings, Dragonborns, Genasi, ect), or less common races (Aasimar, Kenku, Thri-kreen, Warforged, Shifters, Lizardfolk, ect). There are so many options for humanoids.
3. More High CR Creatures
So similarly to my sentiments with more dragons, I wouldn't be expecting a monster manual with nothing but CR 23+ creatures. But I would love to see at least a handful of really high CR creatures that could be used as the big bad or epic end boss battle of a campaign. An Ancient Aboleth that has been hiding under the waves since the beginning of the Prime World's creation. Elder Titans that were so powerful even the gods who created them feared their might. Powerful Primordial Lords trapped and waiting to unleash ruin against the gods who imprisoned them. Elder Primal Spirits that guard the natural world from all Outsiders and their machinations. Liches that have survived eons, gathering all manner of arcane knowledge that has though lost to the ages. There as so many possibilities.
Another thing which has occurred to me is more variants of creatures from campaign sourcebooks and adventures, like you did with the nosforatu, especially those that have been around since previous editions. Like extra living spells from Eberron (although they're in Time of the Frostmaiden as well), or variant grippli (from Candlekeep) or locathah (from Saltmarsh). Those are just a few I can think of now; I'll nees to look through the books and see if others stand out...