It would be super handy if a petal apothecary could have a proper fluid inventory, instead of just the blockstates empty, water, and lava. For one thing, it would make automation or mass flower crafting significantly easier: I was beta-testing a modpack heavily centered around Botania, and it felt really weird to be unable to pump fluid into the bottom. My automatic response to trying to automate the apothecary was to try this; it's much more intuitive than finding blocks which can use items, which can vary greatly across modpacks. (There might even be a functional flower for this, but it's hard to search for—if so, a tip in the book "apothecaries can be refilled with the help of x flower" would do wonders)
It would also allow for modded recipes to use pretty much any fluid without needing to build an add-on creating new block states for every single liquid. With the addition of tagged fluids, the behavior of lava acting as a brazier would also continue to work as it currently does, just with the ability to add other “voiding fluids”. An inventory of 1'000 mB would keep the max size at one bucket, which would make its base behavior the same on the user end