MYM needs more world building/story
The Chimera and the world of MYM in general could be a lot more interesting. (I'm talking about the in-game aspects, not written lore.)
It makes sense for the Chimera to start as a fetus in a tube, but it's boring for it to stay that way. As it evolves, it should slowly become more and more of a grotesquerie. Maybe the changes are subtle, maybe it looks like a Lovecraftian horror, or maybe it has offshoots of what resembles pink spiderwebbing growing and spreading across the glass like a trapped fungus. (It wouldn't be a bad idea to make the Advisors a bit more unsettling as they level up as well.)
The Chimera talks so much (too much, really), its voice is very important, but right now it's rather unremarkable, sometimes grating. You could have made the voice an interesting part of its design as well, having it use a translator and/or a voice modulator or even a telepathic link for which the Custodian can choose their preferred voice (the current female voice, a more emotionless male voice, a very robotic voice; even just two choices would be nice), and if the Custodian turns it off then it's extremely inhuman or an incomprehensible alien language or even a horrible wordless screeching/squealing that makes you want to rip your own ears off.
I know this isn't a story-driven game, but it still feels like a first draft in terms of its world design. If the player looks (and I don't mean reading the codex), there should be more hints and unsettling details hidden around them. Rather than an in-game novel to read if they feel like it, there should be things around the player to discover. Doesn't have to be a lot, even just a few little things that raise questions without answers and give players something to discuss: visual world building, cryptic clues, snippets of redacted documents, incomplete transmissions. You don't have to explain everything (or anything, really; I feel the codex often overexplains).
There's a bit to be gleaned from evolving the Chimera and leveling up the Advisors, but it never changes and players quickly begin tuning out the dialogue (or get annoyed at its frequency and repetitiveness). The new Hellscape pack has some flavor text that hints at a more interesting world; I hope it's expanded upon in the future.