Wandering Monsters: Through the Vast Gate

Before I talk about the foulspawn and gibbering mouther, I want to point out how odd it is that they do not want to tie the origin of aberrations to the Far Realm because it "might not be a part of every DM's cosmology and campaign", but are totally fine making assumptions about what other planes exist (Ravenloft), how they are arranged (the Great Wheel), and what goes on there (like the Blood War).

Why the favoritism? What about people who think that the Great Wheel is not particularly interesting or creative, or even simply feel that it is not appropriate for their campaign setting? What about official campaign settings that do not include those things? There is no need to make so
many "traditional" assumptions, which among many other bizarre decisions
is something that continues to confuse and frustrate me.



Anyway, on to something less mind-boggling.

Foulspawn
I never used these guys, but there were a few instances where I wanted to (which is a start). My biggest issues are that they just look like ugly, skinless humanoids (gross, but not too interesting), and that despite being warped by the "incomprehensible" energies of the Far Realm can be easily categorized into four distinct types: just cut out the middle man and give us a table of mutations and powers.

Otherwise I like the mix of abilities: teleporting, psychic screams, cascading rage, and reality distorting blasts (which should be able to teleport creatures hit by it, too). I think what would make both them and other aberrants/aberrations even better is to further establish the Far Realm, giving it its own cosmology and/or demiplanes, possibly even a pantheon, just so that you can give them more thematic powers.

Gibbering Mouther
I like the origin of being created from a mass of humanoids. Gives me an Akira vibe, though you could take a page from Fullmetal Alchemist and have cultists sacrifice groups of people to willfully create them. Of course I see no reason tying them more closely to the Far Realm and have them be heralds of some eldritch god, which could also have the added benefit of giving them something to work with in encounters.