Disclaimer: I love this question, but I don't have the PhD to answer it well. I'm going to take a shot at it nonetheless. I won't laugh at me if you won't laugh at me, OK?
We're looking for a way to replace CHON, an acronym for Carbon, Hydrogen, Oxygen, and Nitrogen, the four most common elements in Terrestrial life. Our goal? To create a biome that isn't based on Oxygen (I need to ignore "mild" aka "worse at oxidizing" 'cause, frankly, I'm not even sure if I can make this
Basically, you need a liquid to replace water that isn't based on oxygen. For no reason other than the feel of sunlight on my left cheek, I'm going to try Fluorine.
As a test, H2F (fluoronium) is an acid. From a ridiculous POV, so is water (the "universal solvent"). Humans don't dissolve when they drink water, so let's assume a creature that won't dissolve when drinking fluoronium.
So, let's take on glucose. All creatures need energy, right? Can I make a glucosy something using fluroine? I'm guessing the basic problem is the CH2OH part of the molecule. What is X for XH2FH? I'm thinking phosphorous. PH2FH.
And I'm not going to take on proteins 'cause if by now you think I'm doing anything other than pulling interesting associations out of an etherial hat... I'm so far out of my depth... But I'm having fun, so ppfffffhpt!
So, now we have PHFN. My spidey-sense is telling me we can't use Nitrogen with Fluorine, so we need something a bit more adept: let's use chlorine! (It actually works well with Fluorine. I think...) That gives us PHFCl and, I'm guessing and hoping... the building blocks of life...
On a planet not dissimilar to Venus. Hot and acid prone. The creature would be much heavier than humans (heavier molecules), and would likely favor yellow over red for the basic wine of life. (You know... blood...)
- It is an organic oxidizer
- It is gaseous at room temperature
- It exists on my planet vs. oxygen in the same ratio that oxygen is to fluroine on Earth.
- The molecules it creates would be heavier, which is good for lighter gravity.
And though students of chemistry and the very Angels in Heaven are probably weeping over this answer (or laughing, it might be hard to tell the difference with all the tears), I give you my favored candidate: