I've recently started using barrels a lot more as I'm a Tailor on a Server, and therefore using a lot of barrels for leather, cloth dying, etc. Here are a few suggestions I have in regards to making barrels easier to use.
1. making quicklime, borax, and other recipes involving instantly converting water to another liquid via powder/item.
Currently, to make a solution of quicklime, you MUST, ratio out both the water, and the lime to the radio inside the recipe book and only then will the water in the barrel convert to quicklime. I suggest that when an item is placed in a barrel and the stack size is larger than the recipe required, the game converts the water to quicklime, and the lime powder is subtracted leaving any extra unused in the recipe in the slot to be removed or used later.
2. Greater Seal-ablity and Movable Barrels.
Option to seal and unseal barrels that don't have a sealed recipe to allow for moving them around.
buckets stack to 4, so if barrels aern't stackable, it means that you only gain 10 Liters per slot by putting the fluid into a barrel. As shown by this lovely stock image, there are sizes of barrels which people carry on their shoulders, and barrels of similar were often used on ships for drinking water and other fluids.
Mods such as A culinary Artillery, and Food Shelves, provide greater fluid storage, however you can't move the tun if it has fluids, and bottles are useful, but don't have the volume to be used for industrial purposes.