I believe this has been discussed before, but you know the saying. They would ideally be able to go anywhere you can put a crate (or perhaps in a slot with type "barrel"). Could be filled with a bucket by clicking, and would only hold one type of fluid. Capacity would be configurable in JSON. Should show how much is in it when hovered over.

It has been discussed. The two issues are 1: Forge's constant fluid system changes. And 2: Not having a model of a barrel.

Also, I will NOT have fluid by mouseover. That's a PITA to do. I leave that functionality up to WAILA and other mods that mess with rendering. Other implementation details are vauge, as I'm not the hugest fan of messing with liquids.

Implemented in 13.1.0.