Description
I saw that reactions will be data driven in 1.0 and so wanted to suggest this. I have thought of two ways to do this:

1. Recipes could include a way to add items/fluids as input (alongside specifying what molecules go with it???) instead of just molecules (which I think is how it's getting done right now), and maybe the output could be done like this also.
2. There could be a way to add molecules to items/fluids in datapacks. Maybe make tags for every molecule and items could be registered into those?

Problem This Addresses
Would make it easier for modpack developers to add compat between different mods.

Minecraft Username
Amronos

Items are already possible; fluids will be possible in 1.0 with automatic fluid -> Mixture of molecule conversions whenever you pump fluids into a Vat or mix them in a Basin.