Yeah there would be serious dangers to creating a synthetic biome, but theoretically it should be possible to vastly improve through actual engineering instead of natural selection. And it should be easier than doing any genetic engineering on a human genome, because you could create a more simple model for a single celled biome with specific input and output chemicals. I imagine LLMs could help with creating the map of inputs and outputs, then generate candidates to refine. But I understand we still don’t even have a scientific model for a single cell.

Like produce everything a human needs just from sugars, fats and some minerals. That’s not the way to do it but in an extreme case you could. Maybe we could even engineer to digest cellulose.

Of course you’d want a quick and easy killswitch and be able to replace a synthetic gut biome it with a natural gut biome again. And you’d want some kind of “predator species” that kills any foreign bacteria, like in any biome.